Hi DrFrank,
As someone mentioned before, the dodder is supposed to be good. As far as I remember Rory's tackle shop in temple bar sells the tickets but you can definetly get them in Tallaght Rod and gun shop(near the national basketball arena). I've havent fished it for a hell of a long time but when I did it was in Herbert park in Ballsbridge. The fishing was ok and a size 16 adams caught me a few small trout. The club stocks it with fish up to the pound. I also went fishing for mullet around ringsend...didnt catch any but there are a heap of them at high tide....might be worth a try.
As a member of the dodder anglers you can fish the two lakes up in Bohernabrena(They are listed in peter o reilly's book). They offer good fishing and the surroundings are nice.
I remember my uncle taking me fishing on the river Camac in clondalkin about 20 years ago when I was up visiting my townie relatives. He got a few nice trout on it that evening. I havent a clue how it fishes now and he doesnt fish anymore. You can still fish it in corkagh park for free as far as I know. Incidentally...there is a put and take fishery in corkagh park if you get desperate

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There is a reservoir in leixlip but I dont know if there are any trout in it.
The Rye water in leixlip is supposed to be a good river but I've never fished it. Peter O'Reilly's books would be worth getting.
Tight lines,
Eddie