We do know that the Celts did have a major influenced over today’s Irish dancing with their Celtic designs. Irish dancing dresses are beautifully designed with Celtic design that date back since the arrival of the Celts.
In Irish history we know that Christianity arrived in Ireland after the Celts which, like the Gaels, changed the local Irish culture and customs. As Christians converted the druids of Ireland into Christianity the earlier form of Irish dancing would have changed too although keep in mind we are talking about a period that spans over hundreds of years so the way people danced in Ireland was not changed over night.