McCoubrey married an Irish trade unionist and moved to Belfast. There, she joined the British Women's Social and Political Union (WPSU), travelling to London as a representative of women in the north of Ireland. She joined the Irish Women's Suffrage Society in 1910, and was an active militant. The theme of self-sacrifice was paramount amongst suffragettes and Margaret McCoubrey claimed that suffragettes were continuing an Irish tradition of violent protest.
At the outbreak of the First World War, she disagreed with the WSPU's orders to cease agitation, and instead founded a branch of the Irish Women's Suffrage Society in Belfast. She joined the peace movement and gave refuge to conscientious objectors. At that time, the majority of women in Ulster perceived pacifism as unpatriotic and female suffrage as unimportant in comparison with the dangers threatening wartime Europe. As a result, only a few suffragists remained active during the War. McCoubrey single-handedly ran a month-long peace and suffrage campaign in Belfast in August 1917, inspired by her belief that ‘a woman looking down on a battlefield would not see dead Germans or dead Englishmen but so many mother’s sons’.
She became general secretary of the Co-operative Women's Guild and in 1922, she was elected to represent the Irish guildswomen on the newly formed International Women's Co-operative Committee, which came into existence at Basel.
The guildswomen took an active part in the work of the Co-operative Society Advisory Council, represented on various deputations to the Belfast Corporation, when subjects such as housing reform, sanitation, the appointment of women constables, the need for the enforcement of the provisions of the Medical Treatment of School Children Act, and the amendment of the Criminal Law Act were discussed.
- Margaret Ward, "Conflicting Interests: The British and Irish Suffrage Movements", Feminist Review, No. 50 (Summer 1995), pp. 127-147
- 'An articulate and definite cry for political freedom': the ulster suffrage movement[dead link]