It is not safe to be dogmatic about how names were pronounced in Old Irish. Both spelling and pronunciation have changed a lot since.
And Bres is about as ancient a name as you can
Foras Feasa ar Éireann, Keating's History written in the 17th Century gives this name as Breas http://www.ucc.ie/celt/published/G100054/
Maybe Dennis King or some scholar of Old Irish will drop by an give us some well founded information...