Although Carroll and Carolan have quite separate genealogical orgins, it might be worth mentioning that tracditionally the spelling of the latter was O'CARROLAN - this spelling can still be seen in some very old cemeteries on the Cavan/Meath border, including a beautiful tombstone (now laying flat) of a CARROLAN chieftian and the wife.
When my ancestor came (involuntarily) to the penal colony of NSW in 1820, the British authorities listed him as CARROLL, when his name was clearly CAR(R)OLAN.
Damian Gleeson, Sydney