Ó Ríordáin, Seán Pádraig (1904–57), archaeologist, was born 17 December 1904 in Monkstown, Cork, son of James Reardon (Ó Riordáin) (1865–1939) and his wife Annie Barry (d. 1930). Initially apprenticed in Cork dockyard, Ó Ríordáin studied at night for Department of Education examinations, which he successfully passed and became a national school teacher. He continued to study at night, and graduated BA (1928) in UCC. He was greatly interested in the Irish language and culture, and was influenced by the teaching of Patrick Power (qv); he developed a lifelong enthusiasm for archaeology. Taking his MA with first-class honours (1930), he went on to win the NUI travelling studentship (1931). This provided him with an opportunity to study archaeological field techniques and museum collections in Britain and on the Continent. During this time, at the suggestion of Professor R. A. S. Macalister (qv) and Dr Adolf Mahr (qv), he began to study halberds, a type of artefact typical of the bronze age; for this work, he was awarded a Ph.D. by the NUI.
After his period of travel, Ó Ríordáin joined the Irish antiquities division of the National Museum in Dublin. In 1936 he succeeded Canon Patrick Power (qv) as professor of archaeology in UCC. In 1943, after the death of Macalister, Ó Ríordáin was appointed to the chair of Celtic archaeology at UCD. Both departments with which he was involved developed new areas of research expertise and were particularly active in teaching; the UCD department was transformed by Ó Ríordáin's work, according to Michael J. O'Kelly (qv). Ó Ríordáin introduced a new generation of Irish archaeologists, including O'Kelly, Etienne Rynne and Ruaidhrí de Valera (qv) to the modern techniques of field archaeology in which he had been trained during his studentship.
Ó Ríordáin was noted for his fieldwork, especially for his eighteen years of excavations carried out at the multi-period site of Lough Gur, Co. Limerick, which elucidated much of its complex history, and formed the basis for still-continuing study. He also analysed Ireland's Roman archaeology, contributed considerably to knowledge of Ireland's early medieval history, and initiated important work on the passage graves at Newgrange; his co-author, the Cambridge archaeologist Glyn E. Daniel, published their study of these sites in 1964, after Ó Ríordáin's death. Ó Ríordáin began digging at Tara in Co. Meath in 1955; his preliminary work on this hugely significant site was the first modern investigation, and (though it remained unpublished for many years) is recognised by contemporary archaeologists as being of lasting relevance. Major discoveries, including an undisturbed passage grave on the site, were announced at the time in newspapers. Ó Ríordáin gave frequent public lectures and talks on the radio; he was one of the first archaeologists from Ireland to appear on British television, featuring the reconstruction of ancient dwellings at Lough Gur. He also appeared on the panel programme, ‘Animal, Vegetable, Mineral’, chaired by Glyn Daniel.
He published numerous papers and excavation reports in various archaeological journals, and his major book Antiquities of the Irish countryside (1942), which has been republished in five editions, provides undergraduates and the general public of the twenty-first century with an unrivalled introduction to Irish field monuments. While he was in Cork, Ó Ríordáin revitalised the long-established Cork Historical and Archaeological Society, as its honorary secretary and editor of its journal. Its membership almost doubled, and the enlarged and redesigned journal enjoyed a much wider readership outside Ireland. He was given an honorary life membership of the Society. Ó Ríordáin's address as president of the Royal Society of Antiquaries of Ireland was published in the Society's 1955 journal; it was an important critical assessment of the current state of Irish archaeology. He was president 1953–6. He was awarded a D.Litt. by the NUI, and was elected MRIA (1936).
Ó Ríordáin married in 1936; his wife was the sculptor and painter Gabriel Hayes (qv). They had one son and one daughter. Ó Ríordáin became ill in the autumn of 1956, and died 11 April 1957 in Dublin, at the age of 52.