Lara, It depends on too many things to give you any idea + basements are not that common in Ireland in Domestic dwellings, so your builder may not be familiar..
we'd need to know, the type soil and the ground/ first floor loadings (how difficult it will be to excavate how much steel/ concrete will be required from lateral and above pressures. And what the water table level is like(same as tanking a swimming pool) and access + what happens to adjoining property etc etc
you would be best to seek professional advice- why not consult the developer and or a independent Quantity surveyor. It could certainly be 1/3 of the of the cost...
also consider its proposed use, as any habitable room needs natural light and a means of escape.
do you just love the area? or why not consider somewhere that has ample space without the hassle..