Lochlainn O'Raifeartaigh was born in Clontarf, Dublin on the 11th of March, 1933. He was the second eldest of a family of three brothers and three sisters. His father, Tarlach O'Raifeartaigh, was a Gaelic scholar and civil servant who, as Secretary of the Department of Education and later, as the first Chairman of the Higher Education Authority, played an important role in the development of university education in Ireland. Lochlainn O'Raifeartaigh's mother, Neans, was the daughter of T. J. Morrissey, a first class mathematics graduate from the Royal University Dublin, who went on to become the Assistant Secretary of the Department of Education. Neans had a keen interest in music and a gift for languages. She spoke Irish, French, German and Swedish fluently, and studied Japanese for over 10 years.

Lochlainn O'Raifeartaigh attended St Joseph's Primary School in Marino, Dublin and later Castleknock College. His stay in Castleknock was enjoyable. He admired the beautiful setting and participated enthusiastically in all school activities. He was a member of the Debating Society, played cricket, and represented the school in athletics. In 1950, O'Raifeartaigh completed school, securing a first place in mathematics in the Irish Leaving Certificate examination and a university entrance scholarship to University College (UCD). He had decided to study science, abandoning an earlier intention to study engineering.

Lochlainn O'Raifeartaigh had an outstanding undergraduate career at UCD, completing a joint mathematics and physics honours course as the best student in his year. One of his interests while a student at UCD, the Irish language, led him to join An Cumann Gaelach (the Gaelic Society). An Cumann Gaelach members from all Irish universities, north and south, met once a year in different parts of the Gaeltacht (native Irish-speaking districts). On one such occasion, in Teelin near Donegal, O'Raifeartaigh met Teresa Donnelly, a student from Queen's University Belfast. They were married in 1958. In addition to her love of languages, Teresa shared O'Raifeartaigh's interests in theatre and hill-walking, and she supported Lochlainn throughout his career. Together they had five children: Conor, Finbar, Cormac, Una and Aoife. Their children were a source of pride and joy to him and Teresa.

After graduating from UCD in 1956, Lochlainn O'Raifeartaigh obtained a research fellowship from the Dublin Institute for Advanced Studies (DIAS) to work with the great Irish relativist J. L. Synge. After a year at DIAS, where he had the opportunity to attend lectures by Schroedinger, and after writing three papers in relativity, O'Raifeartaigh was awarded a traveling studentship to study under Walter Heitler, a former professor at DIAS, in Zurich. The University of Zurich had a distinguished record of excellence in theoretical physics. It was, after all, the place where Schroedinger discovered Quantum Mechanics. After completing his Ph.D. on "Non Local Field Theories", O'Raifeartaigh returned to DIAS as an assistant professor. Shortly afterwards, another turning point in O'Raifeartaigh's career came as a result of a chance encounter with E. C. G. Sudarshan in Bern. In 1963, O' Raifeartaigh was invited to spend three months at the newly formed Mathematical Science Institute in Madras, India, to lecture on group theory. In 1964, while on extended leave from DIAS, O'Raifeartaigh was invited to join Sudarshan's group in Syracuse, USA. Even though Lochlainn and his family enjoyed living in Syracuse, they were keen to return to Europe. In 1968, O'Raifeartaigh was offered a senior professorship at DIAS which he accepted. He returned to Dublin after spending some time at the Institute for Advanced Study in Princeton, USA.

Lochlainn O'Raifeartaigh remained at DIAS for the rest of his life, building an internationally respected group in theoretical high energy physics, and always working at the forefront of the subject. His method of collaboration was to interact and discuss physics at the blackboard. These discussions were not polite, considerate exchanges, but rather exciting, heated arguments which continued until the issues being debated had been clarified. The aim of these discussions was to understand the essence of the problem being considered. O'Raifeartaigh often said the main difficulty in tackling a problem was to properly understand it. These far-ranging, thorough, and open-ended discussions are fondly remembered by those who participated in them.

Lochlainn O'Raifeartaigh had wide interests and was fluent in Irish, French, and German. He was a scholar in the history of his subject and wrote a masterly book "The Dawning of Gauge Theory", a historical account of the quest for a unified theory of the fundamental forces of the physical world. O'Raifeartaigh received a von Humboldt Research Award in 1998 and the prestigious Wigner Medal in 2000 for his pioneering contributions to particle physics. He was elected a Member of the Royal Irish Academy at the age of 29, and was also a member of Academia Europea. O'Raifeartaigh was interested in the international aspects of science and was a co-founder of the Irish Pugwash movement.

Lochlainn O'Raifeartaigh's varied and deep contributions to physics were marked by his passion for clarity and his instinct for uncovering the fundamental features in a problem. His best known contributions are undoubtedly his "No Go Theorem" in 1965 and his work on supersymmetry breaking in 1975. The "No Go Theorem" brought to an end attempts to combine internal Lie group symmetries with Poincaré symmetry in a non-trivial way. This work used methods of group theory which were unfamiliar to most physicists at the time and established O'Raifeartaigh as a researcher of the first rank. The O'Raifeartaigh mechanism for supersymmetry breaking was a result of a request from A. Jaffe at Harvard in 1974 to report on recent work by Wess and Zumino on supersymmetry in Aspen. This made him study the work of Wess and Zumino carefully. Later on, urged on by a remark of Pais that the problem of breaking of supersymmetry needed attention, O'Raifeartaigh quickly showed that supersymmetry could not be broken by a single chiral superfield. Initial attempts by O'Raifeartaigh to show that supersymmetry breaking by Higgs' mechanism was not possible failed, and eventually he was able to show that a minimum of three chiral superfields are needed for Higgs' mechanism to work. A delightful account of the way this work was done is contained in O'Raifeartaigh's contribution to the book "The Supersymmetric World" (Ed. D. Kane). O'Raifeartaigh was a very productive and creative researcher. He published over 200 papers with over 60 collaborators! As new ideas came up, O'Raifeartaigh studied them, discussed them with passion, and made important contributions to them. A few examples of his many contributions can be given to illustrate the range of his interests.

(i) The Effective Potential

In a widely cited paper, O'Raifeartaigh showed in collaboration with Y. Fujimoto and G. Parravicini that the effective potential was a convex function of the scalar field even when the classical potential was not. Since the effective potential was regarded as a "quantum improved" object to be used to study the ground state of a quantum system, this result would seem to imply that any classical spontaneous breakdown of symmetry with their associated phenomena, such as Higgs' mechanism, were canceled by quantum corrections. It also seemed to suggest that the proposal of Coleman and Weinberg that quantum effects could lead to spontaneous symmetry breaking could not be valid. However, a careful analysis of these problems carried out in this work showed that this was not the case and established the validity of spontaneous symmetry breaking and the Coleman-Weinberg mechanism.

(ii) Magnetic Monopoles

The surprising discovery by Polyakov and 't Hooft that magnetic monopoles could appear as solitonic excitations in certain gauge-Higgs field theories started a rigorous programme of trying to understand what was involved in creating such monopoles. O'Raifeartaigh, in collaboration with P. Houston and S. Rouhani, studied axially symmetric monopoles in detail. They found that such configurations have intricate topological features. In particular, they showed that the Higgs field has to vanish on the axis connecting two monopoles of the same charge.

(iii) Toda Theory and W-Algebras

In a remarkable series of papers, Lochlainn O'Raifeartaigh, in collaboration with J. Balog, L. Feher, P. Forgacs and A. Wipf, tackled the problem of understanding the general properties of two-dimensional conformally invariant soluble field theories. It had been found that such theories are realisations of extensions of chiral Virasoro algebra including Kac-Moody algebras and the W-algebras which are polynomial extensions of the Virasoro algebra by higher spin fields. The study of such polynomial extensions was initiated by Zamolodchikov. Later it was shown by Gervais and Bilal that Toda theories, which were integrable lattice theories, provide a realisation of W-algebras. It was furthermore suggested that Toda theories were closely related to Kac-Moody algebras through Wess-Zumino-Novikov-Witten (WZNW) models. These are Lagrangean realisations of Kac-Moody algebras. O'Raifeartaigh and his co-workers established the exact relationship between Toda theories and WZNW models. Specifically, they showed that Toda theories could be regarded as WZNW models reduced by certain conformally invariant constraints. This result allowed them to elegantly show that the W-algebra of Toda theory followed from the algebra formed by gauge invariant polynomials of the constrained Kac-Moody currents and their densities. It also allowed them to obtain general solutions of the Toda field equations from simple WZNW solutions.

(iv) Seiberg-Witten Theory

The construction by Seiberg and Witten of an exact non-perturbative effective action for N=2 supersymmetric Yang-Mills theory created a tremendous amount of interest. Their effective action offered a way to understand quark confinement in this model. The result of Seiberg and Witten was based on the assumption of a certain duality relation. In collaboration with M. Magro, R. Flume, I. Sachs and O. Schnetz, O'Raifeartaigh showed that the Seiberg-Witten effective action was unique and that it could be obtained without using duality arguments simply by exploiting the supersymmetry of this model.

In all his work, Lochlainn O'Raifeartaigh's love of physics, his interest in understanding fundamental problems clearly, and his ability to present results with precision and clarity shone through. He was an inspiring teacher for physics students in Dublin as well as abroad. His lectures on the Group Structure of Gauge Theories, published by Cambridge University Press, are very popular among students as well as researchers. O'Raifeartaigh passed away on the 18th of November 2000 after a brief illness. He is survived by his wife and his five children.