Ireton's contemporaries had no doubt of his central role in the English Revolution. His political enemies and army allies all regarded Ireton as the arbiter of army political strategy, its ‘alpha and omega’. From September to December 1648 Ireton drove the New Model to purge Parliament, the institution that was its political master. In the subsequent six weeks his relentless energy would have been crucial in maintaining the nerve of those who recognised that the monarch had to die. In this, Ireton's influence would have especially told on Cromwell.

As with others Ireton has remained in Cromwell's shadow. This is a product of his early death in Ireland, Cromwell's own importance before 1649 and his pre-eminence after 1651. Although the role of the individual has become unfashionable in academic history the figure of Cromwell necessarily continues to loom large over the historiography of the years 1645–60. A man typically beset by doubt throughout his life, one of those closest to Cromwell, especially in 1648, was Ireton. Ireton's influence on Cromwell would have been a key factor in shaping his response to the failure of Charles I to come to terms.

Clarendon wrote that Ireton ‘was thought often by his obstinacy to prevail over Cromwell, and to exort his concurrence contrary to his own inclinations’. Furthermore, it: