Wikipedia
O'Heerin is an Irish surname meaning "grandson or descendant of Uidhrin" ("little grey"). The O'Heerin clan of Leinster originates in the ancient kingdom of Ui Failghe corresponding to the modern County Offaly, the territory given by the second-century king Cathair Mór to his eldest son Rus Failge.
Uidhrin O'Maelmuire was an 11th-century Ui Neill chief ruling Cinel-Fearadhaigh, now the barony of Clogher in County Tyrone in Northern Ireland. His son and grandson also ruled as chiefs, before Mac Cathmhaoil in the 12th century. It is unknown if these chiefs were related to the Leinster clan. The name next appears with the historian Giolla na Naomh Ua hUidhrín (O'Heerin) who died in 1420. His addition to O'Dugan's "Topographical Poem" commences with "Tuille Feasa ar Érin Oigh" (An addition of knowkedge on sacred Erin).