That does sound very worrying for you 
The aggression could be a result of anxiety and misery which stems from bullying; has that been fully dealt with by the school? My first move would probably be to talk to the school about that, and generally to find out how your DS is getting on socially,
The bullying itself could also be the other children's response to a social-communications problem that came along with (and perhaps been masked by) your DS's advanced intellect, whicht could be making you DS respond aggresively to demands that he doesn't understand or can't meet. If your DS was already starting to act up a bit before the before the bullying then it could be a combination of both.
I would discount boredom as a factor in a UK state school at that age, as most schools are still quite adaptable and a very bright child would be kept busy and engaged. Later on the National Curriculum is much less flexible and boredom is more of a factor. I don't know how it is in Ireland.
In the UK I would be talking to the school special-needs co-ordinator and also to the GP and asking for assessments for development issues, not just giftedness but also for other conditions that might go with a very uneven cognitive profile, such as autism. There are some extremely intelligent children with Asperger's who show no major sign of any problems til they hit school, and who don't show problems at home because home doesn't place the same demands or triggers as school - busy, crowded, noisy, large groups of children, fast-changing, etc.