In the original 1923 Road Lists, the A2 coast road did not actually cover the entire coast of Northern Ireland. There was a gap in east County Down, between Donaghdee and Clough, which was instead originally numbered B11. The gap in the A2 was filled early on with that road taking over the whole of the B11, thus removing the gap in that number and making our road redundant.
Starting where the A2 met the B21 (now numbered A48 here) in the centre of Donaghdee, the B11 headed south along the coast of the Ards Peninsula to Portaferry, where it met the A20. It then crossed Strangford Lough by ferry. It is possible that this ferry precluded its Class I status in 1923 but the A2 now crosses Strangford Lough here on what is the only ferry on an Ax road in the UK.
In Strangford itself the B11 met the A20 and then continued along the coast via Ardglass before turning inland again to reach Clough. It ended where the A24 turned into the A2 on the southern edge of town.