The B125 was a short link road in the County Down coastal town of Newcastle. Owing to the road's short length its history is not fully known. It was originally unclassified but had come into existence by 1961. The road has now been taken over as part of the A2 one-way system through town and is only accessible in the direction described.
The road turned off the A2 on the Central Promenade and headed inland along Bryansford Road. It took the next right turn into Shimna Road, a street name it kept for the rest of its route. With the Mountains of Mourne in the background the road headed north, soon crossing the Shimna River. The now-unclassified B180 Bryansford Avenue was then crossed, with the town centre to the right. A short distance further on the road ended by returning to the A2.