My name was formerly "diggin" and was changed by the FCA (irish army) to "diggins" by their pay office. At the time it was too difficult to get them to change it back and of the two brothers in the army, only one changed it back to "diggin" after they left.
So now our half of the family is all "diggins" ie. 6 brothers plus families.
We all believed that the "diggin/s" name was the same as duggan, deigan, duigan, deegan and various other spellings originally. All of them derive from "dubh ceann" (gaelic for dark hair) as you mentioned before.