Hi,
Garden waste is accepted free at some of the Dublin City Council bring centres (e.g. Windmill Road in Crumlin). This is restricted to domestic users and trailers aren't allowed.
The whole situation is very strange, despite a supposedly common strategy for Dublin waste across the 4 local authorities, there is a wide variation in charges..free in DCC, reasonable in SDCC's Esker Centre, and expensive in DL-R.
p.s. have you tried home-composting? I know it can be hard to accomodate everything you produce, but if some is mixed with appropriate kitchen waste, it can be very worthwhile.