I'm pretty sure it's called The Bag Shop and they now have branches in St Stephen's Green Shopping centre, Westmoreland St and a few other places. I've always found the Liffey St (the street by Ha'penny Bridge) branch the best though as their staff are so helpful. I've bought all my backpacks etc there over the years and they're great.
I just googled them and they now have an [broken link removed].
Well I know The Bag Shop is/was on your left if standing with your back to the bridge but it might have changed. I'll be heading in that direction today or tomorrow so I'll check it out and let you know.