In Irish Names and Surnames by Patrick Woulfe (6) we have;
Ó CARTÁIN – I or IV -- O Cartaine, O Cartayne, O Cartan, Cartan, Carton, Carten, Cartin, Curtan, Curtayne, Curtin, &c.; “des. Of CARTÁN,” if not a corruption of Mac Artàin (q.v.) which seems probable. At the beginning of the 17th century it was found chiefly in the Midlands –Westmeath and Longford—but appears also at Castleisland, Co Kerry, where it is now pronounced Ó Curtàin, and angl. Curtin, &c., as it is also in West Limerick where is is very common. V. Mac Cairteàin.
Ó CÁRTHÁIN – I – Cartan, Carten, (Carthy), &c ; a rare West Connacht surname, the origin of which I cannot trace.
6 Irish Names and Surnames Patrick Woulfe Genealogical Publishing Com, 1993
ISBN 086303816 9780806303819