Every band I've played in - and I've played in several different bands in different musical genres - gets its tempos from a variety of sources. In the case of rock'n roll, there is a widely accepted repertoire of songs and tunes which people dance to. Same in jazz and - I suppose - the same in ceilidhs.

As far as ceilidh bands are concerned, some have drummers and some don't. The process of setting tempos is not as mechanical as writing down BPM or a metronome. In many cases, experience tells you what speeds to play the dances at. And how do you get experience? By playing at dances and watching the dancers! Catch 22! :-)