During his time on the New Jersey docks, Quinlan joined the Longshoreman's Protective Association, part of the Socialist Trade and Labor Alliance, the trade union wing of the Socialist Labor Party of America (SLP). This lead Quinlan to membership in the Marxist SLP itself, in which he was a member of Section Newark, New Jersey. During this time he formed a close political association with radical Irish socialist James Connolly — a key leader of the 1916 Easter Uprising who would be executed by the British government in the revolt's aftermath.