There's an internet café on Talbot St (close to the Gardiner St junction) which advertises left luggage facilities and is open 24/7. Not exactly beside the Bruce gig, but might be of use depending on where you're coming from?
Connolly Station have an automated locker system just on the left when you pass through the main platform exit/ticket checker, beside entrance to the toilets. That's a bit away from the RDS, but would be handy if you're passing through. They take small hand luggage or large suitcases. There's usualy a couple available. I've used them a few times and they're handy.
You'd have to check if you can access them 24/7 though, I'm not sure.