The most numerous class of Irish locomotives was a six-coupled goods locomotive, which was purchased 111 times over a period of 37 years. It wore class number 101 on the GS&WR and was designated class J15 on the Great Southern from 1925. Thanks to their low axle loads, they could also be used on branch lines, where they even pulled passenger trains.

Schematic drawing in original state

Locomotive Magazine, January 1918

The design was by Alexander McDonnell, who was Locomotive Superintendent from 1864 to 1883. Most examples were built at GS&WR's Inchicore works, but a smaller number came from the UK by Beyer, Peacock & Co and Sharp, Stewart & Co. Some of the examples built at Inchicore used components of old 0-4-2 goods locomotives.

Schematic drawing in rebuilt state

Locomotive Magazine, February 1918

All locomotives built up to 1881 had cylinders with a diameter of 17 inches and a stroke of 24 inches. The boiler was initially made of iron and could therefore only be operated at a pressure of 140 psi. Later boilers were steel and could be approved for 150 and later 160 psi. From 1882, cylinders with a diameter of 18 inches were installed.

An attempt to increase efficiency was number 165, built in 1888 with a compound engine with cylinders of 18 and 26 inches. As expected, this was more economical than its sisters, but could not muster the same pulling power. In 1896 it got the same cylinders as her sisters with simple steam expansion. The larger boilers that were installed in the last locomotives built in 1902 and 1903 were more successful. Later, a large number of locomotives were fitted with the same boiler. Many also got superheated boilers with Belpaire fireboxes. 

In heavy goods train use, the class 101 could pull up to 45 loaded cars. Limits were set only by the relatively low adhesive weight on wet tracks. In addition, the relatively small boiler did not have very much power reserve. After more powerful goods locomotives were procured at the turn of the century, the operational area of the class 101 shifted almost entirely to branch lines. Nevertheless, new locomotives were introduced in 1929 and 1934 with the classes 700 (J15a) and 710 (J15b), which were based directly on the class 101. The last copies were not retired until 1963.