The reason is that the ISO codes were inherited, via a long winding path, from the postal system. That's why you'll see that the Isle of Man, Jersey and Guernse

The reason is that the ISO codes were inherited, via a long winding path, from the postal system. That's why you'll see that the Isle of Man, Jersey and Guernsey have codes as well (even though they are not independent sovereign states). And of course Ireland has a code. But then after the postal codes were created came partition, and the Unionists called themselves British, so wouldn't have wanted a code that made them felt less British, and so whereas they would have been under the Ireland code before partition they joined GB after partition.