The Boilerhouse is what you make of it. I haven't been there myself in a long time. In fact, I met my ex partner there in 2000 are we were together for 9 years and remain very close friends.
You ring a buzzer, they let you in and at the desk you get a towel, condoms and sachets of lube. Don't be too drunk getting in or you'll be turned away. I know a lot of guys have a few drinks beforehand for "Dutch courage" to go there but don't get too locked. There are 3 floors, the bottom one has lockers where you strip off and put your towel on. The middle floor has a jacuzzi, steam room and sauna - good for checking other out guys out and cruising.
The top floor has private cubicles where you play. It's busiest at weekends with quite a few younger guys but quieter weekdays with more older men. Eye contact often means you and another guy is interested in a hook up. You can stay all weekend (or used to) getting a pass out with a hand stamp.
It also has a small coffee bar as well where you can get chatting to other guys. There is a small smoking room too. Play safe!
Dublin used to have another sauna - The Dock - but it was a filthy kip and if still there I wouldn't recommend.