Ségéne was apparently one of the chief protagonists in the Irish Paschal controversy. In 632/3 Cummian Foto (qv) of Clonfert wrote an open letter to him and the hermit Beccán on the Easter question. This was apparently in reply to an earlier letter (which no longer exists) from Ségéne and others accusing Cummian and the southern Irish churches, that had after the synod of Mag Léne abandoned the traditional Celtic dating of Easter, of being heretical. Cummian exhorted Ségéne and Beccán to follow the example of the reformers and adopt the Roman observance of Easter. Ségéne and the clergy in the northern part of Ireland, however, did not conform; after meeting in synod, they addressed a letter to the pope seeking clarification on their position. The reply, issued by John IV, and preserved in part by Bede (‘Historia ecclesiastica’, ii, 19), was addressed to Segenus presbyter among others.
Under the long abbacy of Ségéne (623–52), Iona achieved its greatest degree of influence. When Oswald sought to re-evangelise his kingdom of Northumbria, he asked Ségéne to send him a missionary. In 634/5 Bishop Áedán (qv) and a number of companions were duly dispatched to complete the work left unfinished in 633 by the Roman missionary Paulinus. When Áedán died in 651, Ségéne sent Fínán (qv) to replace him and continue his mission. In Ségéne's time, Iona was in control of a monastic paruchia that extended from Pictland to Northumbria and included several foundations in Ireland. Ségéne died in 652. His feast-day was observed by the Columban community on 12 August.