Generally it depends on where the apartment is located.
When I was buying an apartment a couple of years ago, Bank of Ireland basically said that Dublin post codes with single digits would need 10% deposits, everywhere else would be 20% I think. This probably differs between banks and I'm not sure what the situation would be outside of Dublin.
I was able to get a mortgage from KBC for a one bed apartment in Dublin with a 10% deposit as a first time buyer - all other banks required 20% deposit for one bed apartments - that was 3 years ago now though so things may have changed.