McAteer, a dock labourer, was involved with the Irish Citizen Army and the Socialist Party of Ireland.
[1][2] While in the USA he held membership of the Industrial Workers of the World and was jailed for three months in 1917 for draft dodging.
[3] McAteer returned to Ireland to fight in the Irish Civil War for the Anti-Treaty IRA.
On 11 June 1923 McAteer and Jim Phelan participated in a robbery of a post-office, undertaken to gather funds for the IRA.
During the course of the robbery McAteer fired a shot that killed a man and as a result he fled to the USSR.