Geashill is roughly formed from the ancient
Túath Géisille of the
Uí Failge septs of
Leinster. As
Viscount Clanmalier the Ó Diummasach (O'Dempsey) held part of this barony, where the main castle of the clan was located. The Ó hAimherigin (O'Bergin) sept are noted as chiefs in this barony in medieval times. ==List of settlements==