Several people in this very interesting thread have mentioned the importance of competition dancing in formulating the way reels are played in Ireland. Clearly, this was one major influence but the prime influence was the work of the Gaelic League and their attempts of impose (or re-impose) regional styles in Ireland. It was they who decided that there were groups of dances danced to different rhythms which they called the "Clare Set", "Kerry Set" and so on. Most of this was clearly nonsense: largely the romanticised invention of the prominent League members rather than based on historical research. In spite of this, the work of the Gaelic League became central to the way dance came to be regarded in Ireland.

If you want that short story about this have a look at this article in the Irish Traditional Music Archive and the ones that it links to:-
https://www.itma.ie/features/printed-collections/ceili-dancing-the-gaelic-league

If you want the long story then set aside a lot of time to read the 1000+ pages of Reg Hall's utterly fascinating and outstanding book A Few Tunes Of Good Music especially the early chapters. It is available as a computer flip-book at
http://www.topicrecords.co.uk/a-few-good-tunes/

I didn't want to be stuck at my desktop to read this huge book so I took the Topic website's offer of having it as a free download. Firstly, I downloaded it on to my Kindle. I found it almost impossible to read in this format - tiny print with the facility to increase the size of the font blocked, so I would not advise it. Then I downloaded it in the format to be read on my Samsung Galaxy tablet and that I found much more satisfactory.

The ITMA articles present an historical account of the GL's influence. Dr. Reg Hall allows his opinions alongside his thorough and meticulous research. Anyone who knows Reg will soon get to know his strong opinions on all aspects of the tradition, but to my mind his approach to the music is one of the healthiest that I know.