The smallest of Northern Ireland's motorways is the odd little A8(M) - and that's saying something, because the province has more than its fair share of very short motorways with strange histories.
This one was supposed to be the M2. When work started to build Belfast's new main road north, it headed this way, but a late decision was taken to re-route the M2 northwest from Sandyknowes Roundabout instead of turning northeast. The motorway north-eastwards from the roundabout was already under way, so it was left as a stub of motorway branching off the M2 and forming a sort of bypass around the west of Newtownabbey.
If it hadn't been intended to form part of the M2, it wouldn't be subject to legislation that made it a motorway, and nobody would have ever thought it sensible to make it one. It doesn't actually connect to any other motorways - at its southern end, it reaches only the roundabout at Sandyknowes, which is crossed by the M2 on a flyover but is not itself a motorway. At its northern end there are no motorways at all.
The A8(M) isn't completely unique in connecting to no other motorways - roads on the mainland like the M2, A3(M) and part of the A1(M) are the same. It is, however, unique in also having no other junctions between its end points, making this motorway little more than a mile of dual carriageway between two roundabouts.
For reasons lost to history, the road was designed to carry two lanes each way plus hard shoulders, but at some point the northbound carriageway was re-painted so that it had three narrow lanes and a narrow hard shoulder, fitted into the existing carriageway width. Stranger still, the right-hand lane ends halfway along the motorway, the two remaining lanes are pushed over to the right, and a new lane opens on the left. This arrangement doesn't seem to achieve a great deal, but then the joy of the A8(M) is that virtually everything about this mile of nondescript dual carriageway is a bit weird.
Several decades after it opened, this A8(M) was joined by another A8(M) over in Scotland. That one is also a very short dual carriageway between two roundabouts. You may draw your own conclusions.