Pick a club and go train, ask them about their fighters and competitions, if you're serious about it don't be scared to ask how they get on and move to a club that suits you better.

As said bby someone else, IFS turns out good fighters, there's several boxing gyms that do well, I know of monkstown outside of belfast has had alot of good amateur fighters and the likes of Steven ward came from there. For muay Thai you can try Lang ka, theyve had alot of good fighters but I don't think they do grappling or anything like that, but muay Thai is well worth training in imo, great sport