Tabular data about post code data is at MySociety.org:
ONS Postcode Directory (ONSPD), February 2012 edition (thanks dvdoug!). This include full UK (including Northern Ireland) postcode locations.
Those files don't include the headers, which is annoying. This page has a version from a different vintage which had a header added. If it's 47 columns, hopefully they match, but it's taking an age for me to download.
It looks like Northern Ireland has some more complicated licensing rules for commercial use, which may have contributed to that data being harder to find when the Wikipedia pages you're looking at were originally created.
It looks like someone has approximated shapes for the BT postcodes which you can see on this Open Street Map page. Getting those in a useable format is pretty obtuse, as best I can tell.
To conclude, I'm sorry that the original text of this answer was misleading: the source I identified doesn't actually have what you need. You may have to contact NISRA; maybe more requests from the public will chip away at their data publishing process.