These two 2-4-0T side tank locomotives were built by Beyer, Peacock in 1857 and 1858 with the works numbers 53 and 55. The Belfast & County Down Railway gave them the numbers 7 and 8 and used them in passenger service. They had inside frames and were the first locomotives of the company with outside cylinders. The drivers had a diameter of 5 feet and 6 inches or 1,676 mm and the leading axle was fixed in the frames. The firebox was located between the driving axles and had a raised top. Another feature of the locomotives was a Beattie feedwater heater. 

In 1859, No. 8 was renumbered to 6. Around 1870, they received new boilers with flush topped fireboxes. No. 6 was withdrawn in 1894 and its number was directly reused for a 2-4-0 tender locomotive. No. 7 was withdrawn in 1896 and its number was passed on to a 2-4-2T tank locomotive.