E160 per Sq Ft sounds reasonable to me. I am a quantity surveyor and have a good deal of experience in this type of stuff. Generally if asked in the pub “how much would an extension cost?” I usually budget for something like E150 to E175 per Sq Ft. I have seen some extensions go to E195 per Sq Ft.
Obviously this is a budget figure and is very much depending on the extension itself (the layout and floor to wall ratio), your location (Dublin V’s Country), what rooms are you adding (kitchen V’s bedroom), finishes specification, etc.
I would say that the two quotations coming back the same is a bit fishy. It wouldn’t be unheard of for contractors to speak to each other!! Try another contractor from a different part of town of a different town altogether.
One other thing, if you get two quotes for 65K and a third for say 30K, I would be nervous of going with the 30K quotation. The 30K man will have to cut corners somewhere in order to make a few euro for himself.