The problem is, without knowing what the slogan actually was, it's impossible for anyone here to offer a balanced opinion on the rights and wrongs of what happened.
When Irish people get together, especially if drink is flowing, sing-songs are a natural way of them enjoying themselves.
A lot of the songs are rooted in Republican attitudes, but that's a long way from actually offering tacit support to the IRA. It's a bit like a group of people singing 'Rule Britannia' who would be aghast if anyone thought that indicated support for the BNP.
So I would have to reserve any judgement on this, not knowing what has been said, but it does once again highlight the stupidity of posting on social media without due thought of the consequences.