The name dates from medieval times, to the pre-Norman kingdom of Thomond ('North Munster') where the Dál gCais (in English: 'Dalcassian') clan, centred on the regional rulers – the Uí Briain (O'Brien) family – were dominant.
The Crottys were one of eight septs of the O’Briens (i.e. descended via the female line – hence the different surnames).
In common with the O’Briens – and the millions of descendants of the other seven septs – the Crotty sept's likely ultimate common ancestor is Brian Bóruma mac Cennétig ('Brian Boru') c.941 – 1014, and therefore possibly (via the Dál gCais) as far back as Cormac Cas in the 3rd century.
Spelling variations include Crotty, O'Crotty, Crotti, Crothon, Crotton, Crotone, Crottee, Crottey, O'Crottey, O'Crottee, O'Crottie, Croddy, and Cratty.
The other seven related septs of the O'Briens are the families: Bernard, Consadine, Lysaght, MacMahon, O'Mahoney, Padden/MacFadden and Plunkett.