Monsell, Harriet (1811–83), founder of the Sisterhood of St John the Baptist and anglican saint, was born in Dromoland, Co. Clare, eighth child of Sir Edward O'Brien (1773–1837), landowner and MP (Ennis, 1795–1800; Co. Clare, 1802–26), and his wife, Charlotte Smith (d. 1856) of Cahirmoyle, Co. Limerick. Her family was a junior branch of the O'Briens, earls of Thomond; her eldest brother, Lucius O'Brien (1800–72), became 13th Baron Inchiquin in 1862, and another brother was the Young Irelander, William Smith O'Brien (qv). Harriet was educated at home and was an unpromising student. On 21 September 1839 she married Charles Monsell (1815–51), son of Thomas Bewley Monsell, archdeacon of Derry. Charles was studying for the church and the newly married couple moved in spring 1840 to Oxford. After ordination he was assigned to his father's curacy and they lived for a year in Dunboe before moving to Limerick, where Charles was appointed prebendary of Aghadoe. His health was always delicate and from about 1847 he took to spending more time in Italy, though he exerted himself sufficiently to petition his friend, William Ewart Gladstone, to help secure a pardon for Smith O'Brien after the 1848 rising. He died in Naples on 29 January 1851 and his wife, who was exceptionally close to him, determined not to marry again. As she was childless she was free to devote herself to the church, and found her scope almost immediately on a visit to her sister in England in spring 1851. Her sister's husband, the Rev. Charles Harris, was curate in the parish of Clewer, near Windsor in Berkshire, where a ‘house of mercy’ had been set up as a refuge centre for prostitutes in 1849. Its founder, Marquita Tennant, had recently been forced into retirement by ill health, so Monsell refounded the house as a sisterhood, of which she became the first member when clothed as a sister on 29 May (Ascension day) 1851. Two other women soon joined and on 30 November (St Andrew's day) 1852 Monsell was professed mother superior of the new order. She named it the Sisterhood of St John the Baptist, but they were known colloquially then, and remain known, as ‘the Clewer sisters’.
When Monsell took charge of the house of mercy, it held thirty prostitutes or ‘penitents’; within three years there were eighty and they occupied only a small part of the Clewer sisters' work. Monsell was indefatigable and established in Clewer an orphanage, St John's Home; a school, St Stephen's Mission Home; and a ward for convalescents which in 1865 grew into St Andrew's Hospital. The local council refused to sign the contract for it, so she did so herself and then began the necessary fund-raising. In 1860 the community planted their first mission in London at Pimlico, where they soon established a girl's school and an orphanage. Three more London missions followed, as well as ones in Gloucestershire, Torquay, and Devon. The first overseas mission was in New York, founded by Sister Helen Folsom, and the order eventually spread to India and South Africa. Monsell, who is described as humorous and generous, combined piety and a strong emphasis on prayer and meditation with energetic practicality. Her personality and her family position helped attract women of birth into the sisterhood, and she was on good terms with leading politicians and churchmen, including her cousin by marriage, Archbishop Tait.
In September 1875 she had to resign the superiorship as her health was failing and she then settled nearby in Folkestone in a house called The Hermitage, where she died on 25 March (Easter Sunday) 1883. The order continues to flourish and runs the Clewer spirituality centre for retreats, St Anne's Home for the elderly, and St John's Home for mentally handicapped women. In the Common worship (2001), a new anglican prayer book and calendar for the twenty-first century, Monsell was named for the first time among the lists of saints and her feast day is commemorated on 26 March.