With a POD no earlier than the Easter Rising, have Britain attempt to hand over sovereignty of Northern Ireland to the Irish Republic, only to have the Ulstermen attempt to fight Dublin's attempt to assert power.
According to some people on this board, Dublin would not be able to successfully impose its will on Ulster's population due to the weakness of its own armed forces and the fact the Ulster population is trained and armed, but I am not especially knowledgeable about the situation.