Mark, Info I have; Diggins derives from O' Duibhgin, as does Deegan, Diggin, Digan, Deighan, Duighan, Dugan and probably Duggan.
One authority claims three origins; An Clar (Clare), Gaillimh (Galway), and Lock Garman (Wexford). The name is most common in Uibh Fhaili (Offaly). May have been more than one family of origin. The name means Dark (or black) haired...so it's logical that there may may have been several families. Most prominent was in Laois the family estate (2000 acres) were stolen by the Earl of Coote in 1667, Sir Earl was sent to Ireland by ElizabethI to plunder kill as he found profitable.