Two other things.
Two swim caps!
Cheap flip flops to get to and from the water.
There are also showers in a few beach locations including Myrtleville and Fountainstown which are handy.
No stopping ye now!
Yeah, footwear is a must for some "beaches" . Missus bought a pair of kind of wet-suit ankle boots. I think she said 12 Euro in Dunnes, and swears by them. Her feet were always the first thing to get painful in the cold on those bracing dips. I've seen people wearing wet suit type gloves as well.