it was created by a random person
Irish Modern Dance Theatre was created in 1991.
Riverdance is a Irish step dance
No one person can be said to have created Irish dancing. There are many kinds of Irish dancing. It would have developed over hundreds of years, with many people adding new types of dance to what was there.
If what you are asking is What is the word for dance in Irish? It's damhsa [dhowsa] or rince [rink-ye].A dance as a function is damhsa, céilí [kaelee], or rince. For a short history on Irish dance see the link History of Irish Dance.The name for Irish dancing in Irish (Gaelic) is: rince Gaelach.
Jig or Irish step dance
Same reason any dance is - people enjoy it. Irish dance is a very beautiful dance.
I believe it is the Single Jig/Light Jig in Irish Dancing that was once a Irish Peasant Dance.
McConoghmy School of Irish Dance
I think the main purpose of Irish dance is for fun, enjoyment, and entertainment, but people do it for many reasons. It was created in Ireland as a way to celebrate and party, and was carried on to America and other countries. Now people compete in Irish dance competitions and go to dance classes - it's tons of fun. There is no real reason though, because you don't have a reason to dance, you just dance.
a lively Irish dance with three letters is tap dance a jig?
they dance at feises
i call clumsy Irish dance something you need to work on