Ímar Ua Donnubáin or Ivor O'Donovan, and possibly nicknamed Gilla Riabach, was a legendary and celebrated petty king, navigator, trader, and reputed necromancer of 13th century Ireland belonging to the O'Donovan family. He may or may not have been the second son of Cathal, son of Crom Ua Donnubáin, from whom the modern Clancahill dynasty descend. In any case Ivor is the ancestor of the historical O'Donovan sept known as the Sliocht Íomhair or "Seed of Ivor", who are generally considered to have been one of the four great septs of the family before being all but destroyed in the 1560s in a conflict with the Clancahill main line. Although mostly legendary, Ivor is possibly referred to in one or two near contemporary sources.