cóigeadhach Definitions (by source) Show all Poems on the O'Reillys a pentarch, ruler of a province, 1396 Poems on the O'Reillys. Editor: James Carney. Stories from Keating's History of Ireland belonging to a fifth; rí c., king of a province. Stories from Keating's History of Ireland. Author: Geoffrey Keating. Editor: Osborn Bergin.