- The Irish Confederates, a Catholic coalition that initially opposed Cromwell but later allied with him against their common enemy, the English Royalists.
- Ormonde's faction, a group of Irish Protestants loyal to the English Crown who supported Cromwell as a means of defeating their Catholic rivals.
- The Protestant settlers in Ulster, who saw Cromwell's campaign as an opportunity to secure their lands and establish their dominance over the Catholic majority.
- Some of the Old English, the descendants of the Anglo-Norman settlers who had assimilated into Irish culture, who supported Cromwell in the hope of gaining religious toleration and political autonomy.