Against his father's wishes, Neligan also joined the
Dublin Metropolitan Police (DMP) in 1917. Picking up travel documentation from the local
Royal Irish Constabulary barracks, he declined a suggestion that he enlist in this armed rural force. After service as a uniformed constable with the DMP, Neligan became a detective and was transferred to the
counterintelligence and anti-political-subversion unit, the
G Division, in 1919. In May 1920 Neligan's elder brother Maurice (1895–1920), an
Irish Republican Army (IRA) member and friend of
Michael Collins, persuaded him to resign from the DMP. Neligan received word that Collins wished to meet him in
Dublin. He persuaded Neligan to rejoin the DMP as a
mole for the intelligence wing of the IRA. Along with detectives
Eamon Broy and James McNamara, Neligan acted as an agent for Collins and passed on vital information. This included documents about the relative importance of police and military personnel and he also warned insurgents of upcoming raids and ambushes. One author claims that Neligan may have been a double agent working for the British. In 1921 Collins ordered Neligan to let himself be recruited into
MI5, from which he picked up passwords and the identity of their agents. This intelligence was passed to Collins. After Broy and McNamara were dismissed in 1921, Neligan became Collins' most important mole inside
Dublin Castle. ==Irish Civil War==