You do not need a special code. Both area are part of the UK and share the same phone number system.
Dial the International Access Code (00 from the UK). Dial the Country Code (353 for Ireland). Dial the Irish Area Code and Local Number. OMIT the initial zero from the area code.
The country code and area code of Letterkenny, Ireland is 353, (0)74.
The country code and area code of Carlow, Ireland is 353, (0)59.
The country code and area code of Ennis, Ireland is 353, (0)65.
The country code and area code of Clonmel, Ireland is 353, (0)52.
The country code and area code of Portlaoise, Ireland is 353, (0)57.
The country code and area code of Longford, Ireland is 353, (0)43.
The country code and area code of Dundalk, Ireland is 353, (0)42.
The country code and area code of Castlebar, Ireland is 353, (0)94.
The country code and area code of Drogheda, Ireland is 353, (0)41.
The country code and area code of Naas, Ireland is 353, (0)45.