I have just updated the "Isle of Man" codes with the newest data from http://www.npemap.org.uk/ Guernsey and Jersey are already on the islands as far as I can see. The Isle of Man should be correct now.
I don't know much about Northern Ireland. There are about 280 codes in the newest npe dump not yet included in the geonames database. There is some information about Scotland and Northern Ireland on the npe website :
Do you think it would be useful to include the full npe dump in the geonames database?
The problem is that there are no place names in the npe dump and I don't have time to find the missing place names. We can add the outward and inward postal codes from npe and freethepostcode.org in the geonames database and webservices without place names. Or someone volunteers to find the place names for the 280 missing outward locations. What do you think?