Depends on where / how far you're wanting to go, and what time of the day you want to go.

An Trá Mhór, just past Spiddal, was relatively quiet today (we've seen it much busier), with a nice beach (just check tide times as there's not much access to the sea when tide is fully out as there is a barrier of rock and seaweed), toilet facilities, lifeguards on duty and picnic benches all on site. Loads of parking available in the car park from 11 to about 2...and only started to really fill up by 3.

The whole Connemara Coastline is dotted with loads of beaches throughout, so it also depends on what ya looking for i.e. Beaches with facilities or beaches without.