Not that the path the border between Northern Ireland and the Republic of Ireland follows is particularly logical in the first place but it has at least one particularly unusual piece of meandering.
I’m not sure how to get the grid reference, but if you check google maps there is a little enclave (exclave?) of territory from the Republic projecting into the North, situated between Newtownbutler, Clones and Redhill. Actually 54.146427, -7.309804 should bring you to it.
In the space of just a few miles the N54 Cavan Road passes from NI into the Republic and back again.
Is there any particular historical reason for this?