I also have a 3 phone with Skype. I also raised this question. Sorry to tell you but you cannot make any calls to Irish mobiles\landlines from skype on the 3 phone.
If you have a wireless enabled mobile phone you could get a SIP client on your device and use a service like blueface.ie (no affiliation) - I use it at home with my IP phone (cable connected to broadband modem) and use the PAYG option. I also use the SIP client on my wireless Nokia E51. Quality is great, no complaints and you can ring anywhere in the world. http://www.blueface.ie/rates/rates.aspx