Starting on 30th March (judging by trial and error on the Ryanair website), Ryanair will begin daily flights between Belfast and Bristol and from what I can tell, this is in addition to the 4 current destinations thus they will not be replacing any destinations. Good stuff!
By the way, Ryanair are having a cheap flight sale on at the moment thus you may be lucky and get return flights to Prestwick (or Liverpool, East Midlands or Stansted) for £1 each.
I am tempted to take a days spotting at East Midlands however finding a day suitable for both myself and a friend is proving difficult.