Well, they aren't from the same language but they are from related languages. Celt languages today can be divided into Brythonic/British (P-Celtic if you want to sound very science-y ;o) - Welsh, Cornish, Breton and Goidelic/Gallic (Q-Celtic) - Irish Gaelic, Scottish Gaelic and Manx (Isle of Man).
So 'hill' is today in...
Brythonic:
Welsh - bryn
Cornish - bre/bron
Gallic:
Irish - cnoc
Scottish - cnoc
Manx - cronk, knock
Now where the idea that
Brian meant 'hill' came from is beyond me. The official stance of etymology is that it's unknown but *probably* from the Old Celtic for 'high, noble'. I suppose they're assuming a connection to beann/beinn (top, peak) but that's still nothing more than assumption...though if all one wants is a straightforward 'meaning', full-on accurate or not, it'll do nicely.
Devon