The class PP was a 4-4-0 introduced in 1896 by Charles Clifford, originally designed for mail trains between Dublin and Belfast. At this time, they were the heaviest and most powerful locomotives in Ireland. Compared to the earlier class P designed by J.C. Park, they had a larger firebox and a cylinder diameter of 18 instead of 17 inches. The first batch of three locomotives was built in 1896 by Beyer, Peacock & Co. and was also called “Precursor class” after No. 70, the first of the batch. Three more followed in 1898 which initially had cylinders of 18.5 inches. As early as in 1899 they were relegated to slower lines.
Two more were built in 1906 which were also called “Tornado class” after No. 106. These had a boiler pressure that had been increased from 160 to 175 psi. Nine locomotives of another variant were built in 1910 and 1911. These had a Phoenix superheater and two of the batch had came from the GNR's shops in Dundalk. Boiler pressure was initially 160 psi and was later increased to 175 psi. As the Phoenix superheater did not fulfill all requirements, the whole class was rebuilt with new boilers with a Robinson superheater between 1917 and 1930. These were now called class PPS. The last class member was withdrawn in 1963.