Birth registration was only introduced in Ireland in 1864, so you won’t be able to get a birth certificate for your ancestor if born in 1815. You might however be able to view their baptism if you know which parish they live din, and the records go back that far. You don’t say what denomination your ancestor was but I’ll assume RC. RC parish records are on-line free on the National Library site:
http://registers.nli.ie/parishes
Galbally records start in 1810 so you may be OK there.