While the public image of legislative debates is often less than favourable, parliamentary deliberations can be an important indicator of policy preferences, issue saliency and cohesion within political parties. We consider the case of a parliamentary debate that had a considerable long-term political legacy, forging a party system that endured for almost a century. The debates in the Irish parliament over the 1921 Anglo-Irish Treaty were a critical juncture that split a dominant party, resulting in, first, a civil war and, later, a new mode of party competition. We analyse the text of the debates from this period to see if they contribute to a greater understanding of the ensuing split. Few differences between the two sides in parliament are found, which might explain why few were the differences between the key actors in the party system that evolved.