The Cogitosus Life of Brigit is of considerable historical and archaeological interest. On the basis of various similarities with the other Lives of Brigit, especially the ‘Vita Prima’, it is probable that Cogitosus drew from existing written sources which had preserved some traditions of Brigit's life and miracles, but the Life generally reflects the situation at Kildare at the time of writing. It describes Kildare as a double foundation, with one monastery for monks and priests, ruled by a prior, and another for nuns, ruled by an abbess. The first bishop had been Conláed (qv), who was asked by Brigit to become bishop so ‘that he might govern the church with her in the office of bishop and that her churches might not lack in priestly orders’ (Prol. 5). Kildare's importance through her contacts abroad is suggested by Conláed having obtained his archiepiscopal vestments from overseas – Brigit subsequently gave them away ‘for the sake of Christ in the guise of the poor’. The Life makes bold claims for Kildare, asserting that Conláed and his successors were primate of all the bishops of Ireland: ‘the head of almost all the Irish churches with supremacy over all the monasteries of the Irish, and its paruchia extends over the whole land of Ireland, reaching from sea to sea’ (Prol. 4). It states that Kildare was then one of the greatest centres of pilgrimage in Ireland, ‘a vast metropolitan city and the safest city of refuge in the whole land of the Irish for all fugitives, and the treasures of kings are kept there’ (32. 9).
Cogitosus's description of the basilica at Kildare is our only eye-witness account of the structure and furniture of an Irish church in the seventh century. It describes the church as ‘of awesome height towering upwards. It is adorned with painted pictures and inside there are three chapels which are spacious and divided by board walls under the single roof of the cathedral church’, enabling the separation of ‘a large congregation of people of varying status, rank, sex and local origin’ (32. 2–3). The bodies of Brigit and Conláed ‘are laid on the right and left of the ornate altar, and rest in tombs adorned with a refined profusion of gold, silver, gems and precious stones with gold and silver chandeliers hanging from above, and different images presenting a variety of carvings and colours’ (32. 1).
The Cogitosus Life is written in an unpretentious Latin style and is of elementary construction, with no attempt at achieving a consecutive narrative. It seems to be mainly concerned with celebrating Brigit as the instrument of divine grace which is manifested through her miracles, her great faith, and her charity towards the poor. The feast-day of Cogitosus is given in the Martyrology of Tallaght as 18 April.