According to the 1994 Guide to the Royal Canal of Ireland, the smallest lock on the Royal, Lock 18, is 75 feet (22.9m) long, with a breadth of 13.3 feet (4.0m) and a depth of 4.7 feet (1.4m) on the cill.
According to Waterways Ireland, the navigational criteria for the Royal Canal are:
Length: 21m
Beam: 3.9m
Draft: 1m
L T C Rolt, in Green and Silver, said that the maximum size of vessel that could navigate the whole of the Royal was 70 feet by 13 feet 1 inch by 4 feet six inches “(theoretical)”. He said that lock sizes varied considerably, the shortest being 75 feet and the narrowest 13 feet 3.75 inches.
The navigational criteria for the Grand are:
Length: 18.5m
Beam: 3.9m
Draft: 1.2m (1m in Dublin)
So the Royal is throughout shallower than the Grand (except in Dublin) and the locks have got narrower and shorter.
Irish waterways history 