According to this site, the diggins family is a branch and anglicized version of the old gaelic family O'DUIBHGHINN deriving from Ireland. Other variations of the name are Deegan, Doogan, Duffin, Doohan, Duggan and Diggan. Thissite states that the oldest found lineage of this family was king of Munster in 177 AD. The site states "A Kerry branch of the family have anglicised the name to O'Diggin."
A family friend of mine, George O'Leary, who emigrated from Ireland in around the 1980s also says that he knew many Digginses within the Kerry, Munster area. This is pretty promising information if you ask me.