ESB Sportsco (beside the greyhounds in Ringsend) do lessons on Saturday mornings for non-member kids. Ours have been there for the last few months and have come on a lot. I can't remember how much it was, but I remember being surprised that the amount they quoted was for the entire 10 week period and not per-month, so I think they are quite reasonable. There was no queue, no hassle, just turn up and start whenever the next 10 week session started.
I saw the queue once for Bluepool (Monkstown) signup and almost laughed myself silly - all these parents lining up outside the pool one Saturday morning fretting about whether they would be lucky enough to be admitted in order to be able to get their name on the 'possible' list, not even to get a guaranteed place. Seemed like a crazy situation.
Marian College also do lessons as far as I know, but I don't know what age or what time.
z