Hope you can make the changes with more happy than painful memories baldyman27. If you google "charity shops in Dublin" you will find quite a number of shops in the Dublin area. I don't know if any of these charities will give you a set time for collecting the clothes, or if in fact you need to have them collected - but you could give them a ring and ask.
If you fear that your sister or your goodself might find someone else wearing your mam's clothes perhaps you might consider giving to a charity that doesn't have too many shop outlets. In that way they would probably be only distributed in one area. I am biased, being a Mater Hospital patient, but I usually give my no longer needed clothes to the Mater Hospital shop in Dorset Street. (some way from Rathgar). Again, it's only my personal choice. If it would help you, there might be a charity that would have struck a note with your mate mam.