Cairell (d. 819), son of Fiachnae and overking of Ulaid, belonged to the Cenél Demmáin lineage of Dál Fiatach. His father, Fiachnae (qv) son of Áed Rón (qv), died as overking of Ulaid in 789. Cairell had at least two brothers, Eochaid his predecessor in the kingship and Loingsech (d. 800), airchinnech (lay–abbot) of Dún dá Lethglass (Downpatrick). Eochaid may have been responsible for moving the dynastic seat from Dún dá Lethglass to Dún Echdach (Duneight, Co. Down). In 809 Cairell challenged his brother Eochaid for the kingship and defeated him in battle; Eochaid, the annalist tells us, escaped. On Eochaid's death the following year, Cairell succeeded to the kingship. His reign witnessed increased viking activity along the north-east coast; in 811 a raiding force was successfully repelled. Cairell, however, faced mounting opposition from his brother's sons. In 819 he was defeated and slain at Lapast in Carn Chantain (possibly the townland of Lappoges in the parish of Dromore, Co. Down) by his nephew, Muiredach son of Eochaid. The subsequent kings of Dál Fiatach came from Eochaid's line, and it is they who feature in the royal pedigree. The descendants of Cairell (Uí Chairill), along with the progeny of his other brother Loingsech, filled ecclesiastical roles at Dún dá Lethglass, even if they did not necessarily aspire to the ascetic standards of Cairell's first cousin, the saintly Diarmait (qv) grandson of Áed Rón, founder of Dísert Diarmata (Castledermot, Co. Kildare).
AU; AFM; Bk Leinster, i, 193; vi, 1451, 1469; O'Brien, Corpus geneal. Hib., 322; M. C. Dobbs, ‘History of the descendants of Ir’, Z.C.P., xiii (1921), 330; xiv, 82; Byrne, Ir. kings, 124, 285; NHI, ix, 132