as well as a continuing great interest in Irish Trad, I have also taken a keen interest in English Trad, in particular the dance tunes…One thing raised in my study is that , as far as English dance tunes are concerned the use of a number of tunes one after the other seems to be a relatively new idea…tunes for dancing were generally played one at a time for a particular dance…many of the tunes themselves being linked to one particular dance. My question is , is it the Irish tradition that medleys, or sets have always been the way to play them or is that a relatively new idea…,3 , 4 or more reels played one after the other, same with jigs etc…or was it ‘tradition’ to play the tunes singly for a dance ? Les.
Dunno. The folks I know play ITM for ceili dancers as multi-tune sets in order to keep from falling asleep.
Just kidding, although I do think that sort of thing is more for the musician’s benefit. I can’t count how many times dancers have said they don’t even notice that the tunes change during their dancing. I’m sure that’s not true of everyone, of course.
There was a disscusion on thesession.org back when it was not a place for recorder players 
about this. I think it was determined that sets area session invention and bakc when every body played for dancers they just played the same thing over and over again.
sets of tunes basically developed for set dancers yeah.
can think of the galway set, the connamara set, the kilfenora set. course there’s loads. the sets become so common that its almost inevetible when one of the tunes is played the whole set gets hammered out then.
newer sorts of sets have emerged more or less based on cds from bands or individuals. eg. michael o raghallaigh has a set on the nervous man cd, sport and the hawthorn bush. if playing the first in a session, it usually only takes a nod to know that the second will be played.
the whole thing has a tendancy to take on a life of its own though, gets complcated.