Maybe thats what you're looking for :
http://www.nisra.gov.uk/geography/default.asp6.htm
from their site:
"The Central Postcode Directory (CPD) has been compiled for users by NISRA Geography and contains both current and terminated postcodes in Northern Ireland. It is however freely available only to all Northern Ireland Mapping Agreement (NIMA) participants or those working on behalf of NIMA participants."
The data are pw protected, but they're free. To receive the password you must agree at something the ireland's statistical wants.