Probably be 50-50 as they are both saying the same thing, though he might have a stronger case as he has a witness. Are you sure she is telling you the truth? Maybe she just don't want to get into trouble.
Also, who was in which lane? In my experience, car in the left lane frequently drive "straight" through the roundabout, cutting into the right hand lane....it's much harder to do if you are in the right-hand lane as you have to swing out quiet far.
With the way the irish use roundabouts....I'm surprised this doesn't happen more often!