Impossible to say without drawings.
What size is the garage?
Can the existing walls be used or do they need to be replaced with proper structural walls and foundations.
Is there a proper floor slab in place or just poured concrete? This will have to be lifted out to be replaced if its just a poured concrete slab as with new habitable space you will have radon, insulation, damp proof and sub floor fill to consider.
Typical Dublin house - 50k. If this scares you, get drawings and some quotes. If this is what you thought it would be, then proceed to get survey, drawings and quotes.