Write an article based on this "Select all the objects that are part of the same group, duplicate them (ctrl+d) and unify them (Path->Union). Cut the border that delineates Northern Ireland so that it stops at the ocean and doesn't follow the coast of Northern Ireland."
The duplication is necessary to keep the original regions separate for one part and unify the other. Once you have this new object, you can remove the fill or give it a very transparent fill and a thicker border. Look at the image, showing the result of having done this with Northern Ireland and the entire island. Place the object showing the complete island is now in a lower layer (use the "End" button on your keyboard), so that the border only shows when it crosses the island. Create a shape that covers the shape of the stroke you wish to eliminate and then use the tool Path->Cut Path. This will divide the border into two pieces so that you can erase (type delete) the border you don't want.