Portugal Telecom (PT WIFI) has hotspots installed in Caldas da Rainha at the Post Office and McDonalds. Cost varies between Euro 5 for an hour and Euro 50 per week.
As far as I know there is no city wide wifi available at present. Check with you ISP in Ireland to see if they have a reciprocal arrangement with and ISP's in Portugal.
As far as I am aware, at present, the Portuguese ISP's offering mobile wifi (Vodafone, TMN, Kangaru and ZAPP) only offer such services on a 12 month contract sign-up on on a reciprocal roaming basis.
If you have a phone line where you are in Caldas you could subscribe to the SAPO ADSL Free option which essentially is pay on demand.
Details here:
http://adsl.sapo.pt/free.html
Hope this helps.
