It's quite difficult in Ireland to guess about churches in circles. With the amount of circles still left I doubt there were that many. Several are built next to circles or tombs though.

One of the main problems is that many early churches are in round enclosures, but these were not circles. For many years old raths and hillforts were (and still are) seen as being the home of the Gentry. When the monks arrived they were given some old raths to build their churches in. As I mentioned before the kings didn't adopt the new fangled religion straight away and I think they were taking the piss a bit by giving the monks these raths hoping that the little folk would drive them off. It also served the practical purpose of giving a clear limit to the amount of land that the monk could have - there are many tales of monks performing miracles to increase the amount of land they could obtain. One such is St kevin at Glendalough. The king told him he could have as much land as a goose could fly around in an hour (or somesuch) ... and gave him a lame goose. Kevin healed the goose and had most of the king's land! The goose then became Kevin's constant companion. As a small note, Kevin has many animal/nature stories associated with him. I reckon he was one of the first Druids to convert to Christianity and still had many of the 'old ways' at hand.