McCullagh was born in Dungannon in Northern Ireland in 1874. He worked as a correspondent for the New York Herald. In 1903 he was living in Japan working for the English language newspaper The Japan Times. Observing the growing tension between the Empire of Japan and Russian Empire, he studied the Russian language and moved to Port Arthur, the major Russian military base in Manchuria in 1904. Obtaining a post for the Novi Kraï (New Land) newspaper of Port Arthur, he became a non-military observer embedded within the Imperial Russian Army at the start of the Russo-Japanese War . At the war's end, he was evacuated in March 1905 as a prisoner of war, traveling from Dalny to Ujina on the Nippon Yusen liner Awa Maru. His experiences were published in 1906 as With the Cossacks; Being the Story of an Irishman who Rode with the Cossacks throughout the Russo-Japanese War. He subsequently returned to Russia to cover the Siberian Intervention during the Russian Civil War and, at one point, was captured by the Bolshevik Red Army. He also covered the Spanish Civil War in 1937. McCullagh died in White Plains, New York in 1956.