My surname is Killen. Both my father's parents were called Killen. My grandfather's family is from county Down in the Northern province of Ulster. My grandmother's Killens came from Ennis in county Clare in the West of Ireland. At great grandfather level both surnames were pronouced "Killeen", now adays Killen is pronouced "Kill-in".
In the county Down the name was first recorded after the colonisation as O'Killen but it is accepted by scolars that this is incorrect as many of the surnames listed in records of the time frequently confused Macs and Os. When I looked at the variation of Killen spelling in county Down were i live there was a clear pattern of where Killen, Killin, Killing, Killion,Killeen occured, there were also McKillens, McCullens,McKillions, McCallions,McCullions,etc.
In some parishes in county Downthe change in names could be tracked. I have a genealogy site at www.caora.net for county Down which may be of interest if you can isolate your family in that area. I no longer have time to support the site but I keep it active as it tends to get some traffic.
If you find a copy of Edward MacLysaght - The Surnames of Ireland in paperback it would be worth having a copy as it is more or less the standard text on Irish surnames.