He evidently visited many churches founded by Patrick or associated with him, perhaps retracing the routes of some of Patrick's missionary journeys and stopping at churches to enquire about their founders, their archival records (some of which may have been in the form of proto-charters), and visible remains from Patrick's time. He claims to have seen a psalter written by Patrick himself ([II 3] 5), an inscription on stone in Patrick's hand (30, 2), and three square patens made for Patrick by Assicus (qv), the saint's copper-smith (56). Tirechán's written sources seem to have mainly consisted of a text held by Ultán (liber apud Ultanum) and a narrative (plana historia), probably a primitive Life of Patrick. He also made use of information provided orally by Ultán and other informants. Some parallels with the Muirchú (qv) Life of Patrick suggest that he and Muirchú used versions of a common source.
The Latinity of the ‘Collectanea’ is pedestrian and the structure of the text does not conform with normal hagiographical convention. Instead, it is arranged to represent Patrick's various itineraries, which are presented as taking place mainly in Meath (among the Uí Neill), in Connacht, and in Leinster and Munster. The author's perspective on the itineraries suggest that he was writing in Connacht, probably in the vicinity of Cruachu (Co. Roscommon). The purpose of the ‘Collectanea’ was to provide a dossier of churches founded by Patrick which could be used to legitimise Armagh's claims to an extensive paruchia. The claims made by Tirechán for the pan-national character of Patrick's mission confirm this, though the author seems to have been aware, albeit in a confused fashion, of a separate mission by Palladius (qv).
His references to outbreaks of plague (‘great plague’ (8), and ‘the most recent plagues’ (25)) correlate with mentions of plagues in the annals: the great cholera (?) plague of 664–5, which decimated the Irish monastic population, the bolgach or smallpox outbreak of 680, and a mortalitas puerorum (a mortality of children) in 683. It has been suggested that Tírechán was writing in the 680s and that the unfinished state of his compilation was due to his sudden death in that decade. His ‘Collectanea’ survives only in the early ninth-century Book of Armagh, which also contains, among other texts, the only surviving copy in any Irish manuscript of Patrick's Confession.