The purpose of this study is to investigate how premarital co-habitation is transforming the institution of marriage in Ireland. I conducted forty-one in-depth interviews in Athlone in 2007. The sample comprised cohabiting couples with plans to marry, cohabiting couples with no plans to marry, as well as couples who married without living together first. Respondents also filled in an event history calendar, recording key events in their lives, since the age of 16 years. Using a life course analysis methodology, the findings make a major contribution to the debate in sociology over the dynamics of change in the transformation of the institution of marriage. Interview thematic analysis and event history calendar information demonstrate that it is the interplay between structural constraints and individual decision making in relationship development, and how that plays out in people‘s lives that produces innovative family formation patterns, such as premarital cohabitation. The wider societal changes associated with modernisation have created a paradox at the level of individual lives. This ‗marriage paradox‘ means that factors, such as education, career opportunities, flexible family formation patterns which tend to liberalise relationship development and create more committed informal relationships, can result in the deferment of the formalisation of those relationships through marriage. This is a paradox because marriage continues to be the ideal outcome of relationship development for most people. Although there are various pathways to marriage in the current, fluid social environment in which we live, the institution of marriage was still highly normatively valued for respondents in this study. Being ‗ready‘ to marry often coincided with self-actualisation through other life goals, but emotional satisfaction within the relationship is crucial. Emotional satisfaction is now on an equal footing with self-actualisation. Marriage as a social institution is valued in a different way. Periods of co-habitation are entered into as a means of trying to preserve that institution. Instead of marriage becoming de-institutionalised, or losing its intrinsic social status, it is instead becoming re-institutionalised. This study identifies the new social values and norms on which marriage is now based.