When Maude fell ill in late 1939 and was assigned to Pitcairn Island, Gallagher was appointed officer in charge of the three atolls selected for development.
He was assisted by Jack Kimo Petro, later characterized by archaeologist and historian Tom King as "a half-Tuvaluan/half Portuguese engineer and artisan of considerable skill and energy."
The young British official skillfully settled an early, hotly disputed debate among them by suggesting that instead of using the traditional Gilbertese boti system, each household be given a place in the maneaba, or local meeting house.
Gallagher, who by now had been affectionately nicknamed "Irish" by some of the settlers, chose Nikumaroro as the colony's government centre and moved there in late September 1940.
Although the gathering war interfered with shipping, Gallagher and the settlers were persistent, starting work on the government station and an official rest house by manually clearing away many rocks and tree roots.
The most memorable building is said to have been the rest house, with its sweeping thatched roof and wide veranda, complete with a modern RCA console radio in a wooden cabinet (powered by large batteries).
That day he had fallen seriously ill at sea with tropical sprue, an infection sometimes aggravated by poor nutrition which interferes with the small intestine's ability to absorb nutrients, resulting in symptoms related to malnutrition.
They were on the island because during the height of the Battle of Britain in October 1940, Gallagher, a licensed pilot, had radioed his superiors in Fiji to inform them he believed a work party of Gilbertese colonists on Nikumaroro had found a sextant box along with the skeletal remains possibly belonging to Amelia Earhart, an aviator who disappeared in 1937.