If I understand correctly, it was the bishop who had the power to define parishes. Not sure what factors he would have weighed.
I understand that in1678, the diocese of Leighlin was put into administration of the Kildare bishops and later amalgamated in no small measure because the revenues of both places were considered very low.
I wonder if a similar thing could have happened with parishes.
It is really quite confusing to hear the old names of parishes. Sometimes they seem to change the name entirely, and I wonder why. Maybe, the location of the church moved. Or of the PP.
Basically yes. The pre-reformation civil parishes were typically far, far too small to support a single priest via voluntary contributions, so Catholic parishes were perforce much larger, typically the size of 3-6 or more civil parishes (depending on their size).
The same process was evident in the established church, the Church of Ireland. In much of the country there weren't enough parishoners to support having a church in each civil parish, and after disestablishment in 1871 there was the same issue of voluntary contributions. So again, the solution to unite parishes into "unions". The differences being that the CofI did respect the boundaries of the civil parishes in forming these unions, and that (pre-1871) the Viceroy authorized and changed the unions.
Finally as for the names of Catholic parishes, they were named for the location of the Church. So as the church location changed, or the head parish church was changed as between multiple churches in a large parish, so the name of the parish changed.