I can't say for sure why they said it, but since all kids who are new to a country and school are prone to being bullied, and schools should be alerted to watch for it, I wouldn't assume "prone to bullying" meant anything more than just that - at least not until I had some information to contradict all the reports that she was happy and well-liked in Ireland.
Either way, I'm not inclined to see any of this as a problem with her, but with her abusers. Nor am I inclined to look for excuses for them. YMMV
If your child had no issues with bulling, would you be informing the school your child was "prone to bullying?" Of course we don't know if that report is correct, but I don't think this is something people would be informing the school about if the child had no issues with bullying before the move.