I'd say most trad music is not played specifically for dancers at this stage so this is a bit of a red herring (unless you are actually playing for dancers).

Tempo does vary (among other things) with regional styles - Clare is usually fairly steady. As you move up the west coast towards Donegal things tend to get faster. I'm not personally a fan of very fast sessions, but a lot of people find really fast playing exciting and mistake it for virtuosity. Because of this bands tend to play fast on recordings, others learn from these recordings and a whole viscious circle gets going.

Sometimes when I hear people playing really quickly, I wonder if they really like the music they are playing - or are they trying to get the tunes finished as quickly as possible.

Mostly I'd advise musicians to develop their competence to be able to play tunes at a reasonable speed. Once they are competent in doing that they should consider an appropriate speed for whatever set of tunes they're playing - mostly I think a steady pace is preferable, but it's not for me to say. As a backer in a session my job is to back the tune at the speed it is started, although if I think people are overdoing the speed, I will generally say so. Discussing things like this generally works better than trying to impose 'rules'. For example its useful to point out to beginners with a limited repertoire of tunes that if they play the tunes they know at very high speed that they will run out of tunes they know very quickly.