[3] He was born in Eskerbuoy, near Carrickmore, County Tyrone, one of five children, to Bernard McCartan, a farmer, and Bridget Rafferty (died 1918).

He emigrated to the USA as a young man and became a member of Clan na Gael in Philadelphia, Pennsylvania and edited the journal Irish Freedom.

McCartan contested the by-election in South Armagh for Sinn Féin but lost out to the Irish Parliamentary Party candidate.

In late 1920 McCartan outlined (in a formal protest sent to the US State Department) some of the atrocities being committed by British troops in Ireland.

They persuaded Éamon de Valera to support the Philadelphia branch of Clan na Gael against the New York branch led by John Devoy and Judge Daniel Cohalan in their struggle to focus the resources of the Friends of Irish Freedom on Irish independence rather than domestic American politics.

[8] McCartan then negotiated with the Soviet Union in 1920–1921 in an attempt to have it recognise the Irish Republic, at a time when both were pariah states.

As the Minister of External Affairs in the new coalition government, his party leader Seán MacBride put his name forward, with fellow Ulsterman Denis Ireland, to be nominated by the Taoiseach John A. Costello to Seanad Éireann.

[1] McCartan was an admirer of National Socialist ideals and an active supporter of the pro-Axis in front organisations such as Irish Friends of Germany.