It's been a few years, but I had some cut in a place on Capel Street. I can't remember the name, but I searched the Golden Pages and found a listing for AJ Upholstery. That may be the place I used - it might be wise to give them a ring before you go. Contact details are here:
[broken link removed]
If you're looking for just the sponge then there is a shop on Capel St. opposite AJ Upholstery who do upholstery supplies and sponge filling. It would be near to Brertons Jewellers.