Joseph Mallagh was born in County Monaghan in 1873. He graduated in engineering from Galway University in 1896. In his early years he worked on water schemes at Downpatrick, Portadown, Banbridge and Boyle. In 1905 he was appointed engineer to the Sligo Harbour Commissioners and developed a tidal model of the harbour from which significant benefits were derived.
In 1916 he was appointed ‘Engineer in Chief”’ to the Dublin Port and Docks Board. During his period he was responsible for the construction of deepwater berths and extensive dredging. He also introduced automatic signalling for lighthouses and was one of the first to adopt echo sounding for the hydrographic surveying.
He prepared the Parliamentary Bill for the replacement of the Swivel Butt Bridge over the River Liffey with a fixed span bridge and also a new transporter bridge further downstream. The new Butt Bridge was completed in 1932 but the transporter bridge was not built.
Mallagh, as Engineer in Chief, also acted in a consultative capacity to other port authorities. He retired in 1940 and set up his own consulting firm. He remained a senior partner until 1957.
He was President of the Institution of Engineers of Ireland from 1930 to 1931.
He died in 1959.