After Edward A. Watson became acquainted with the Star class at the GWR, he designed the class 400 for the Great Southern & Western based on it. Similarities included, above all, the 4-6-0 wheel arrangement and the four cylinders with simple expansion. In 1916 only the prototype with the number 400 was initially built in Inchicore. Its problem was that the frames were too thin and there were inadequate bolsters for the steam pipes, which often led to them breaking. In addition, the consumption of water and coal was too high.

From 1921 onwards, further locomotives were built, which received some improvements. Together with others from Armstrong Whitworth, the number rose to ten. However, since these were not entirely convincing, the seven remaining locomotives were rebuilt between 1927 and 1937 with two larger cylinders. Some were almost completely rebuilt, while the changes to others were limited. In this form they were successful, so that the last one was used until 1961.