No. It means he is from the UK. United Kingdom of Great Britain and Northern Ireland. Northern Ireland is in the UK politically. It is in Ireland geographically. It is not in Britain at all.
please lets not start 'the troubles' again on esb if youre born in NI you can choose your nationality and the majority consider themselves british, which was why the partition happened its all here: http://en.wikipedia.org/wiki/Partition_of_Ireland
And how did the people who consider themselves british manage to become a majority, i believe you should write the account of Chichester of what they did to the natives on their arrival.