Born on November 23, 1845 at Cahirmoyle, County Limerick, O’Brien is the younger daughter in a family of five sons and two daughters. Her father is William Smith O’Brien, the Irish nationalist and her mother is Lucy Caroline, eldest daughter of Joseph Gabbett, of High Park, County Limerick. On her father’s return in 1854 from the penal colony in Tasmania, she rejoins him in Brussels, and stays there until he comes back to Cahirmoyle in 1856. On her mother’s death in 1861, she moves with her father to Killiney, near Dublin, and is his constant companion until his death at Bangor, Gwynedd in 1864.
From 1864, O’Brien lives at Cahirmoyle with her brother Edward, caring for his motherless children until his remarriage in 1880. Having been hard of hearing since childhood, by 1879 she has become entirely deaf. She goes to live at Ardanoir near Foynes on the River Shannon, and spends time writing. She becomes a staunch supporter of Charles Stewart Parnell.
A bad harvest in Ireland in 1879, combined with Irish political turmoil, causes many Irish people to emigrate to the United States. In articles and letters to newspapers and reviews, O’Brien exposes the awful conditions that exist in the Queenstown (Cobh) lodging houses, on board the emigrant ships, and in the dock slums of New York City, where the Irish have to stay upon landing. A notable piece she writes is the Horrors of the Immigrant Ship which appears in The Pall Mall Gazette on May 6, 1881.
A visit to Queenstown, the port of embarkation, and a tour of the White Star Line‘s Germanic leads her to successfully lobby to get a Catholic priest aboard the emigrant ship to help ease the passage, at least spiritually. That achievement captures even more public attention by virtue of the fact that O’Brien herself is Protestant. Despite the limit of 1,000 passengers, she notes the steamer has carried as many as 1,775 at one time.
O’Brien presses the Board of Trade for greater vigilance, and in April 1882, founds a 105-bed boarding house at Queenstown for the reception and protection of girls on the point of emigrating. The O’Brien Emigrants Home at The Beach, Queenstown fails because it is boycotted by other boardinghouse keepers and local merchants, forcing her to order provisions from Cork.
O’Brien also daily visits three or four of the ships for which her lodgers are destined along with a medical officer. She makes passages herself to America, using the occasion to investigate shipboard conditions and lobby for the reform and enforcement of health and safety standards.
O’Brien finds little effort to provide food, drink or accommodation at the Castle Garden entry facility. She also finds that often the illiterate young women are being tricked into prostitution through spurious offers of employment. Additionally, she notes the high infant mortality rates in the tenements where the women live. She proposes to Archbishop John Ireland of Minnesota an information bureau at Castle Garden, a temporary shelter to provide accommodation for immigrants and a chapel. Archbishop Ireland agrees to raise the matter at the May 1883 meeting of the Irish Catholic Association which endorses the plan and votes to establish an information bureau at Castle Garden. Ireland also contacts Cardinal John McCloskey, Archbishop of New York, about providing a priest for immigrants arriving at Castle Garden.
The Mission opens on January 1, 1884 with Rev. John J. Riordan appointed as the first chaplain at Castle Garden. Immigrant girls needing accommodation are placed in local boarding houses until May 1 when a Home for Immigrant Girls is opened at 7 Broadway. In 1885, the James Watson House at 7 State Street is purchased from Isabella Wallace for the Mission of Our Lady of the Rosary for the Protection of Irish Immigrant Girls to serve as a way station for young immigrant women. Between 1884 and 1890, the Mission provides assistance to 25,000 Irish immigrant women.
In 1881–82, O’Brien embarks on a campaigning lecture tour in the United States. She encounters problems, however, particularly given her Protestant background and the need to enlist support from Catholic clergy. Poor health, and her profound deafness cause her to curtail her activities in America. When she returns to Ireland in 1883, she finds herself suspected of being a British agent whose Emigrant Boarding house and whose plans for an American home for Irish immigrant girls facilitate the government’s assisted emigrant scheme. Supposedly, this would be the scheme that helps landlords clear their estates of poor tenants. In fact, O’Brien opposes assisted emigration, but she continues to assist those who are sent to her.
O’Brien retires from active public work in 1886, moving to Ardanoir, Foynes, on the Shannon Estuary. She spends considerable time in Dublin, where she socialises with Douglas Hyde and the painter William Osbourne. She joins the Roman Catholic Church in 1887. She dies of heart failure on June 3, 1909 at Foynes, and is buried at Knockpatrick.