Ellen Lucy or Nelly O'Brien (4 June 1864 – 1 April 1925) was an Irish miniaturist, landscape artist, and Gaelic League activist.

O'Brien died suddenly on 1 April 1925 whilst visiting Dermod at 66 Elm Park Gardens, London.

The 1906 Oireachtas na Gaeilge featured a number of her paintings, and in the same year she became honorary secretary of a newly established art committee.

[1] O'Brien was an early member of the Gaelic League, being present at its first oireachtas in 1897, and founding the Craobh na gCúig gCúigí (Branch of the Five Provinces).

In 1905, she wrote a long letter in defence of Douglas Hyde and the Gaelic League in the Church of Ireland Gazette.

She held meetings of Craobh na gCúig gCúigí in her flat at 7 St Stephen's Green every Saturday night in 1907.

Acting as a representative for the Gaelic League, O'Brien travelled to the United States with Fionan MacColuim in 1914 to 1915, to fund raise and promote Irish art and industries.

[4] O'Brien noted that she initially thought that the 1916 Easter Rising was "in the nature a demonstration against conscription as it had been announced that the volunteers would resist disarmament".