Yes of course dance music must have a strict rythm which can easily be conceived of as fitting into bars- I suppose what was behind my comment (which probably went too far) was:
- the playing of airs, which defy barlines (I know this thread was about dance music though)
- some early recordings of Irish music in which the rythm does not fit into any modern conception of Irish dance idioms, for example the flute recording of a slip jig where, though the second part follows the usual 3+3+3 slip jig grouping the first part exhibits a Bulgarian-like 2+2+2+3 grouping. I have one recording of Northumbrian piping exhibiting exactly the same rythm.
- the existence of irregular “bars” in Appalachian fiddling. These reels, often recognisably coming from an Irish or Scottish original, however often have an irregular structure. Was this irregularity created in Appalachia, or was it originally present, and later lost in Ireland?
About uilleann pipers starting tunes, most of the pipers I hear start jigs in particular rather slowly with a strongly accented beat, then speed up about halfway through the A part.