Much of the Erne system around Cavan & Gowna, consists mainly of roach and Hybrids these days, since a disease had a big affect on the bream a few years ago (although they are starting to come back, I beleive).
For the past 4 years, we have fished the L/Derg area in C.Clare and the bream fishing on Derg itself has been excellent.
On the Shannon fisheries site, there are contacts that will advise you based on the time of year you are fishing. TJ Obrien covers the bottom end of Derg and also runs a service where he will drop you on an inaccessable shoreline using his boat and he'll put you on fish. I know they take big catches in Scariff bay and up the Scariff river during May and early June.