A Cork native, initially London correspondent of William O'Brien's Cork Free Press, subsequently its final editor, though himself a separatist, personally admired O'Brien.

The paper suffered closure in 1916 soon after the appointment of Lord Decies as Chief Press Censor for Ireland. Decies warned the press to be careful about what they published. Such warnings had little effect when dealing with such papers as the Cork Free Press. It was suppressed after Gallagher accused the British authorities of lying about the conditions and situation of republican prisoners in the Frongoch internment camp.

Gallagher worked alongside Erskine Childers on the Republican publicity staff and fought alongside Éamon de Valera during the Irish War of Independence. He would write several short stories for de Valera under various pseudonyms. Gallagher served long stints in prison due to his IRA involvement and went on many hunger strikes (the shortest lasting three days, the longest 41).

He was subsequently de Valera's director of publicity and editor the The Irish Press in 1931 and was appointed deputy director of Radio Éireann in 1936. He would later serve as the director of the Government Information Bureau from 1939—48 and again through 1951-54. Gallagher has composed numerous short stories, biographies and historical pieces.