According to research by historian Catherine M Burns, O'Brennan arrived in America in October 1914 and stayed there for longer than expected due to the difficulty of transatlantic travel during the First World War.

Working as a journalist and lecturer, O'Brennan traveled to California and Oregon and spoke to women's organisations as an authority on the Gaelic league and Irish art and culture.

Fearful that the Irish republican movement in the United States might be perceived as Irish-run, radical, and un-American, Harry Boland opposed their efforts.

He and Éamon de Valera attacked O'Brennan, deeming her and Kelly as too un-American to work for the benefit of the as yet unrecognised Irish Republic in the United States.

[6] During the Irish Civil War, de Valera used O'Brennan's socialist reputation to approach Soviet foreign minister Georgy Chicherin while in Switzerland.

Her personal papers are held in the National Library of Ireland along with those of other members of her family including Éamonn Ceannt, his wife Áine and her sister Lily.