Mandatory address fields

Hi,

In Ireland we do not have zip or post codes, and the city is not always required. If these are not entered within shipping (and I guess billing) then the checkout process cannot proceed. How can I change these defaults?, this will annoy the hell out of my customers if this cannot be changed and a such is not compatible with the Irish market.

Ideally the fields that I would like to have as mandatory are:

Address Label

Street1

State

Thanks.

Martin.