Try skype - probably a lot cheaper than using your landline let alone a mobile.
To call Ireland from the US you need to use the international access code, which it seems is 011, then the international dialling code for Ireland, which is 353, and then the Irish number without the first zero. (If you see an Irish number of only six or seven digits, then it is missing the local area code, which you will also need to add.) You may be getting confused because lots of Irish sites list a number beginning with 00 - this is the international access code when calling abroad from Ireland or indeed from any other EU country, but seemingly not from USA. So the Tory Hostel no. given on the Oileán Thorái website is 00 353 (0)74 91 35920 - but I think from the US you will need to ring 011 353 74 9135920.
If you're using Skype it will sort out the international access code and country code for you and you just need to put in the Irish number.
Hope that helps!