Denis Dowling Mulcahy (1833 - September 1900) was a leading member of the Irish Republican Brotherhood and a medical doctor.
He joined the staff of the The Irish People, which was launched by James Stephens on 28 November 1863, with financing from the IRB in the United States. The offices were established at 12 Parliament Street in Dublin. His colleagues on the paper were Charles Kickham and Thomas Clarke Luby, while John O'Leary was in charge of the editorial department. O’Donovan Rossa and James O’Connor had charge of the business office, with John Haltigan being the printer.
James Stephens entrusted to Luby a document containing secret resolutions on the Committee of Organization or Executive of the IRB. This document would later form the basis of the prosecution against the staff of the Irish People.
On 15 July 1865 American-made plans for a rising in Ireland were discovered when the emissary lost them at Kingstown railway station. Superintendent Daniel Ryan, head of G Division of the DMP, with this and additional information, raided the offices of the Irish People on 15 September and arrested the staff. Mulcahy was tried and sentenced to a term of penal servitude. He served his term in Portland and Millbank Prisons.
On being released Mulcahy went to France, where he lived for two years, and then came to the United States. After many wanderings he settled in Newark, and engaged in the practice of medicine. He died there in September 1900.