Cavan seems to be particularly good at organising water safety classes for young people and Annagh Lake is another location where these classes are run. When we were there a number of children were undergoing instruction. We had a swim later on and there is a nice soft feel to the lake water as you swim out.
Located on the road from Cavan to Enniskillen this is a handy place to stop for a quick dip. There is a decent sized car park, picnic table and a good sized grass area for lounging in the sun/rain/mist.
From Cavan town take the N3 toward Enniskillen. About 6/7 kilometres beyond Cavan you come to the 1st turn left signposted for Annagh. Ignore that turn(!) and a couple of kilometres further on you will come to the car park for the swimming area.
Ordnance Survey Maps No: 27 and 34