an early member of cumann na mban, in 1914 macmahon was the local secretary of the central branch and was secretary nationally in 1916. she also trained the other women in the branch in first aid and nursing as well as related branch duties. she was on the executive committee for cumann na mban from 1915 to 1919. she was an organiser for the o'donovan rossa funeral, a mass demonstration arranged when the body of the fenian was returned from america for burial. macmahon was selected by kathleen clarke as one of the women to deliver messages around ireland for leading up to the easter rising. once the rising began macmahon delivered the mobilisation orders to the cumann section leaders and also delivered guns, both hidden in her bicycle basket. she continued to act as courier throughout the rising, moving between garrisons, such as the four courts, all week. she was estimated to have made more than 50 trips out of the general post office.