Hi Ellen - Irish birth certs start from 1864 (Church of Ireland marriages from 1845).If your gran was definitely from Limerick then you could apply for her birth cert giving all the details- name,approx year,religion,parents names(maiden name of mother)to the Registrar in Limerick.If you are successful you would discover whereabouts she was born. You could try for your grandad too.
Each cert costs 6-98 euros(Isent 7euros cash)
If you have no luck-maybe the county is wrong you could try the main Registrar in Dublin-this takes a lot longer as they are very busy. I have found them all very helpful (could even be Northern Ireland).
Maybe you could then apply for the marriage certificates of your great-grandparents as you have their names. They( gp+ggp) could possibly be on the 1901 census of Ireland, Scotland or England.
Here's the address-
M.W.H.B. (This is Limerick County - different address for Limerick City)
Health Centre
Newcastlewest
Limerick
Ireland
Tel (061) 62545
This is the link - click on 'fees' to get the addresses etc .of all the different offices in Ireland.
http://www.groireland.ie/I hope this helps
Regards
Neptune