He was called by Irish writers Maelseachlainn Ua Cadhla, by John Colgan Queleus, and erroneously by Thomas Carte, O'Kelly.

[1] Ó Caollaidhe became a student at the College of Navarre in Paris, and there graduated with a Doctorate of Divinity.

Following the death of Florence Conroy, he was appointed the archbishop of Tuam on 28 June 1630 and consecrated at Galway on 10 October 1630 by Thomas Walsh, archbishop of Cashel, with Richard Arthur, bishop of Limerick, and Boetius Egan, bishop of Elphin, serving as co-consecrators.

During the Irish Confederate Wars, he raised a body of fighting men in Galway and Mayo, and joined the forces of Sir James Dillon, near Ballysadare, County Sligo.

The Earl of Glamorgan's agreement with the Confederate Catholics and a letter from Charles I of England were found in his pocket.

[1] He wrote an account of the Aran Islands, printed in Colgan's Acta Sanctorum Hiberniæ (p. 714), and is translated in James Hardiman's edition of Roderic O'Flaherty's Description of West Connaught.