In this article, we investigate the effect of political parties on legislative behaviour. We compile and analyse a unique dataset of all roll-call votes for all the sessions of the Irish Free State Seanad (the Upper Chamber in the legislature, 1922—36). The development of legislative parties inside the Irish Seanad led to the formation of cohesive voting blocs after 1928. This caused dramatic shifts in the rank-ordering of members and changed the coalition pattern. The establishment of disciplined parties also sealed the fate of the legislature when the Fianna Fáil party, hostile to the Free State Seanad, became an organized force inside the institution.