Medieval Ulster genealogists describe him as rí Érenn ocus Alban (king of Ireland and Scotland), and quote from a poem, now lost, which has him receiving tribute from Munster, Connaught, Skye and the Isle of Man.
[4] Báetán is said to have forced the king of Dál Riata to pay homage to him at Rinn Seimne on Islandmagee near Larne, modern County Antrim possibly in 574 or early 575.
In 575, at Druim Cett, these two met and made an alliance, fostered by the future Saint Columba, a member of the Cenél Conaill like Áed, to oppose Báetán's attempts to increase his power by extending Dál Fiatach influence beyond the isle of Ireland.
Báetán was married to a woman of the Ui Tuitre (a tribe of the Airgialla west of Lough Neagh in modern County Tyrone) with whom he may have had an alliance.
[11] Don Carleton suggests that the character King Bagdemagus or Bademagu, who features in some continental Arthurian romances, has his origins in Báetán mac Cairill, and that the boar hunt described in the tale of Culhwch ac Olwen in the Mabinogion is an allegorical account of a military campaign fought against Báetán mac Cairill in South Wales.