Thornton
Thornton—In the most parts of South Connacht and North Munster this name is the anglicised form O'Draighneain, a South Galway sept; Drennan and Drinan in the districts about Tuam; in Sligo Dreinan, and Drinan in the district of Oughterard. Likewise Meenagh (Muimheach) in North Sligo, O'Monaghan (O'Muineachain) in Co. Mayo, McSkeghan and McSkean in the districts of Tyholland, Tydavnet and other parts of North Monaghan; correctly written Mac Sceacháin, from Sceach, a thorn-bush.
Alphabetical Index of Anglicised Surnames in Ireland
See also Woulfe’s Irish Names and Surnames
… and O'Hart's Irish Pedigrees