Simms, Mercy Felicia (1915–98), social crusader, was born 3 March 1915 in a nursing home at 89 Lower Baggot St., Dublin, only child of Brian James Gwynn (1883–1972) of 48 Hollybrook Road, Clontarf, civil servant employed as assistant secretary in the labour exchange, and his wife Mary Caroline, eldest daughter of Lewen Burton Weldon, clergyman, successively canon of Salisbury cathedral, England, and Christ Church cathedral, Dublin. In later years the family resided at Templehill, Terenure, Dublin. After 1922 her father's civil service career continued in the Department of Industry and Commerce, where he rose to the post of superintending officer in the establishment branch. Her paternal grandfather, John Gwynn (qv) (1827–1917), clergyman, distinguished biblical and classical scholar, and regius professor of divinity at TCD, married Lucy Josephine O'Brien, elder daughter of Young Irelander William Smith O'Brien (qv). Members of the Gwynn family through several generations achieved distinction across a wide range of endeavour. From her father – who after early retirement from the Royal Artillery was a post-first-world-war pacifist and philosophical socialist – she derived lifelong commitments to pacifism and social justice.
Educated at the quaker school, Rathgar, and at boarding school in Wales, she was a scholar of TCD, and took a BA with a double-first moderatorship in Italian and French, specialising in medieval literature, particularly Dante (1937). After further study in Italy (1938–9), she lectured at TCD in Italian literature (1939–42). She married (September 1941) George Otto Simms (qv) (1910–91), TCD dean of residence (chaplain), whom she met at a Student Christian Movement conference in Dungannon; they had three sons and two daughters. Resigning her lectureship to attend to domestic concerns, after combination of her husband's Trinity chaplaincy with the office of chaplain-secretary of the Church of Ireland training college for primary teachers, she lived with her growing family in the college premises, Kildare St., Dublin (1943–51). She accompanied her husband throughout his clerical career as dean of St Finbarre's cathedral, Cork (1951–2), bishop of Cork (1952–6), archbishop of Dublin (1956–69), and archbishop of Armagh (1969–80).
In all these locations she demonstrated deep commitment to the alleviation of social problems. Active in Church of Ireland social-service bodies, in the 1950s she raised awareness of corporal punishment, especially in the schools. All-Ireland president of the Girls’ Friendly Society, and world president in 1964, she emphasised the efficacy of international cooperation in pursuit of women's rights and equality. Despite the pre- and post-conciliar conservatism of Dublin's catholic archdiocese, through the 1960s she worked alongside her husband to promote the spirit of ecumenism through her involvement with the Irish School of Ecumenics, inter-faith prayer meetings, and pursuit of a common agenda around social issues. Associated with initiatives to alleviate the housing crisis, she first supported the ecumenically based Dublin Housing Aid Society, and then the more radical Dublin Housing Action Group. Involved in educational and civil rights efforts on behalf of travelling people, she was a member of the government advisory committee on itinerancy, addressing, in a significant ecumenical gesture, a catholic social study conference on the issue. George Simms's election to the see of Armagh coinciding with the outbreak of serious civil disturbance throughout Northern Ireland, she participated in discussions of the Irish Association, a forum for cross-border and cross-confessional dialogue. The couple's ecumenism, and their respect for the Irish language and culture, aroused hostility among sections of Northern Ireland unionism. Her grasp of Irish nationalism, derived from her family background, often guided her husband in his evolving perspective. An ardent member of the Irish Anti-Apartheid Movement, she nurtured contacts with such South African church leaders as Bishop Desmond Tutu. Remembered for her acute intelligence, personal warmth, and hospitality, despite a natural shyness and reserve she was fearlessly outspoken. After her husband's retirement from the see of Armagh, the couple lived in Dublin, where she died 1 June 1998.