[1] Sullivan was a member of the Home Rule League, supporting Charles Stewart Parnell in the 1880 general election, being "convinced that without self-government there could never be peace, prosperity or contentment in Ireland".

Sullivan represented a number of constituencies in the House of Commons of the United Kingdom of Great Britain and Ireland.

[1][2] He owned and edited a number of publications (The Nation, Dublin Weekly News and Young Ireland).

His daughter Frances was an Irish-language activist in Craobh an Chéitinnigh, the Keating branch of the Gaelic League (Conradh na Gaeilge) and a lecturer in Irish.

His daughter Anne (who had sixteen children) was the mother of politician Kevin O'Higgins, one of the dominant political figures of the 1920s.