William Tennent (1759–1832) was a successful businessman and banker, who made his mark in Belfast as one of the city’s richest men. He was also a father and, later, a husband. By the time of his marriage in March 1805, Tennent had fathered at least thirteen illegitimate children, with at least four women. The child he would have with his wife, a daughter named Letitia, would be his only legitimate heir. Through this series of illicit sexual relationships, William Tennent created a complex family unit that consisted of legitimate and illegitimate children, half-siblings, step-siblings and step-parents, all of whom were united through a network of unmarried mothers. The example of the Tennent family therefore offers historians the unique opportunity not only to extend knowledge about the making of the family in Ireland, but also to refine ideas about contemporary attitudes to illegitimacy. Using the Tennents as a case-study, this article furthers understanding of the family in Ireland by considering the horizontal relationships which characterised family life, drawing attention to how legitimacy, as well as gender, social rank and birth order, shaped ties between parents and children, and between siblings.