Drawing on largely unexplored archival and published sources in Ireland, France, Britain, United States of America, Canada, Germany and Nigeria, this dissertation presents the first transnational survey of Irish missionary activity in Nigeria from 1885 to 1975. Four missionary orders – two females and two male – are used as a window on the varied aspects of Irish missionary work in Nigeria: The Missionary Sisters of the Holy Rosary, the Medical Missionaries of Mary, the Congregation of the Holy Spirit (Holy Ghost Fathers/Spiritans) and the Society of African Missions (SMA). Five chapters contextualize the evangelical mission of the four Irish societies within the pre-existing African Traditional Religion, Protestantism, and Islam in Nigerian societies. The dissertation analyses how the missionaries’ Catholicism and Irishness influenced their colonial encounters with the Nigerian peoples and the British colonial administration. It explores how Irish missions used education, health and welfare provision as evangelisation strategies and assesses the Nigerian reception to them. This dissertation provides Nigerian perspectives on culture, indigenous knowledge systems, midwifery, community development, acculturation, ethnocentrism, inter-group relations, superficial absorption of foreign religions, and resistance. Finally, it investigates the influence and legacy of Irish missionary endeavour on the people of Nigeria and Ireland. This work argues that the intricate trajectory of Irish missionary endeavour in Nigeria has been characterized by triumphalism, racism, pecksniffianism, defensive spirituality, anti-colonial struggle, social power, and intellectual insecurity. It challenges a historiographical tradition that tends to neglect the contributions of the British colonial administration, Nigerians, and international organisations to the material progress of Irish missions in Nigeria. A study of this nature draws on and contributes to the histories of missions, religious identities, history of medicine and diseases, education in sub-Saharan Africa, welfarism, religious pluralism, ecumenism, gender and sexuality, Nigeria-Ireland relations, and church-state relations.