Re: How old is the surname ?
Blayney originated in Nth Wales before moving to Ireland. The name came from Blaenau meening "the start of a river".
The towns of Blaenau and Blaenau Ffestiniog are located in Nth Wales and are at the start of a river. The Welsh connection die out and all existing Blayneys came from the Irish link.
I do not know the dtes of the original Blayney.