From wikipedia:
The Parliamentarian conquest was brutal, and Cromwell is still a hated figure in Ireland.[4] The extent to which Cromwell, who was in direct command for the first year of the campaign, was responsible for the atrocities is debated to this day. Some historians[5] argue that the actions of Cromwell were within the then-accepted rules of war, or were exaggerated or distorted by later propagandists; these claims have been challenged by others.[6]
The impact of the war on the Irish population was unquestionably severe, although there is no consensus as to the magnitude of the loss of life. The war resulted in famine,[7][8][9][10] which was worsened by an outbreak of bubonic plague. Estimates of the drop in the Irish population resulting from the Parliamentarian campaign range from 15 to 83 percent.[11] The Parliamentarians also transported about 50,000 people as indentured labourers.[2] Some estimates cover population losses over the course of the Conquest Period (1649–52) only,[12] while others cover the period of the Conquest to 1653 and the period of the Cromwellian Settlement from August 1652 to 1659 together.