I was under the impression that BA pulled out of Ireland because they were sick of EI and FR's low fares. The story goes that due to BA's incredible operating costs and were loosing money hand over fist and realised that they would still get feed from EI into LHR anyway, so they "left the Irish carriers to fight it out".
This is a pattern that has continued ever since, look around DUB, ORK and SNN, other than Angel/Harps and Shamrocks there is not a lot of variety.