the union can take a claim under the "trade dispute" claim provisions of the Industrial Realtions Acts. Its a voluntary process but depending on the employer, they may agree to go (public service and large employers like banks usually do go).
Apart from that unless it was pregnancy related or on discriminatory grounds there is nothing you can do. A wrongful dismissal claim can be taken to the Circuit or High Court but that is pointlessly expensive and likely to lose. I doubt its discrimitory given that nothing was mentioned on that front in your post.
he should have received payment for untaken hols and his notice. If he didn't have a written contract, he can claim before the Rights Commissioner for 4 weeks under the Terms of Employment (Information) Act.
All in all, I think i'd be saying, 10 months, move on, leave it.