The service began life in the mid-1920s following the Anglo-Irish Treaty, when the Office of the Directorate of Intelligence was established as the intelligence arm of the new Irish Army. Its codename G2 referred to an army intelligence branch. As the service expanded to include the Naval Service and Air Corps, it became J2 - 'J' for joint services. In July 2025, after recommendations from the Commission on Defence Forces, it was renamed the Irish Military Intelligence Service. Its headquarters are understood to be split between McKee Barracks on Blackhorse Avenue in Dublin and the Department of Defence Headquarters in Newbridge, County Kildare. Intelligence and language training takes place at the Defence Forces Intelligence School - formalised in 2025 - on the Curragh in County Kildare. The number of employees is classified. So is the budget. The only publicly known funding figure is the 'Secret Service' budget for paying informants, shared with the Garda Crime & Security Branch: two million euro in 2024.

During what Ireland called The Emergency (the rest of Europe called it the Second World War), G2 operated under a peculiar set of constraints. The state was officially neutral and non-belligerent. The Defence Forces had to defend the island against potential invasion from any direction. And yet G2 had quietly built secret cooperation agreements with British MI5 and the American Office of Strategic Services - the precursor of the CIA. Listening stations across Ireland intercepted German naval and aerial communications, and the results were shared with the Allies. All thirteen Nazi spies sent to Ireland were arrested - including Hermann Goertz, who lived undercover for nineteen months before being caught. In April 1943 a G2 mission flew by flying boat to neutral Portugal under cover of the Irish Red Cross. Their target was Leopold Kerney, the Irish minister in Madrid, whom Dublin had begun to suspect of being too close to Germany after he met with the Waffen-SS officer Edmund Veesenmayer. After contact with British intelligence in Lisbon and Madrid, G2 concluded Kerney was in fact neutral. The mission read like a thriller. The conclusion was almost an anticlimax.

In 1970, an Irish Army intelligence officer named Captain James Kelly became the centre of a national scandal. The Arms Crisis exposed an unauthorised covert operation to smuggle weapons to the Provisional IRA in Northern Ireland, using fifty thousand pounds diverted from a hundred-thousand-pound humanitarian fund set up to help refugees fleeing The Troubles. Two cabinet ministers - Minister for Finance Charles Haughey, who would later become Taoiseach, and Minister for Agriculture Neil Blaney - were sacked over the operation. Captain Kelly was forced to resign. All three were tried, and all three were acquitted when the case collapsed. The Arms Crisis remains one of the strangest episodes in the history of the Irish state - a moment when the official policy of the government collided with the unofficial sympathies of some of its ministers, and military intelligence found itself caught in the middle.

Declassified intelligence files released after thirty years show that during the Cold War, the Director of Intelligence Colonel L Buckley briefed the foreign minister in November 1983 about the possibility that British or NATO nuclear weapons might be stored at underground facilities in Benbradagh mountain in County Derry. Buckley complained that he lacked the monitoring systems to confirm the suspicion. A separate 1985 file shows G2 carrying out reconnaissance on a British Army installation at Forkhill, County Armagh, after rumours circulated that it was being converted into a nuclear facility; the findings 'strongly discounted the possibility,' but noted that Forkhill was now on a microwave communications network with a possible NATO nuclear function. Since the September 11 attacks, IMIS has worked closely with British MI5, MI6, the American CIA, and reportedly Israeli Mossad. From 2006 to 2014, intelligence operatives were on the ground in Afghanistan, Iraq, Syria, Lebanon, Israel, Sudan, Ivory Coast, Liberia, Kosovo, and Bosnia-Herzegovina. The service remains, in the words of one observer, one of the most secretive intelligence agencies in Europe. The Irish government rarely acknowledges its existence.