In October 1914, she was appointed to a lecturership in mental disease in University College Galway, a position she retained until February 1943.

Through her contacts with people like Thomas MacDonagh, Patrick Pearse (who had once tutored her in the Irish language), Arthur Griffith and Liam Mellows, her belief in Irish nationalism grew and her rational and passionate arguments in its favour had a profound influence on the future Bishop of Clonfert, Dr. John Dignan, who arrived in Ballinasloe in the same year as English.

She began by stating her opposition to the position of the British monarch in the agreement: I credit the supporters of the Treaty with being as honest as I am, but I have a sound objection to it.

[6]She was also one of the few speakers to voice her opposition to the partition of Ireland:[6] The evacuation of the English troops is one of the things that are being held up to us as being one of the very good points in the Treaty.

That is what my constituents told me they wanted when they sent me here, and they have got it.She also rejected the claim made by male supporters of the Treaty that women were opposed to it for emotional reasons:[6] I think that it was a most brave thing to-day to listen to the speech by the Deputy from Sligo [referring to Alexander McCabe] in reference to the women members of An Dáil, claiming that they only have the opinions they have because they have a grievance against England, or because their men folk were killed and murdered by England's representatives in this country.

I should like to say that I think it most unfair to the women Teachtaí because Miss MacSwiney had suffered at England's hands.She stood again for the National University of Ireland at the 1922 general election for the Third Dáil but lost her seat,[5] being succeeded by the independent William Magennis.

She assisted the Anti-Treaty IRA during the Irish Civil War and reportedly served with Cathal Brugha in the Hamman Hotel in Dublin in July 1922.

Along with other members of the Second Dáil who opposed the Treaty, she played a part in Comhairle na dTeachtaí during the 1920s which saw itself as the true government of the Irish Republic.