You have to look at events of Drogheda and Wexford in the context of the wider Wars of the Three Kingdoms. Unlike in the continental Thirty Years War in England the First Civil War had not given rise to much in the way of atrocity. read more
For details of Cromwell's bloody record in Ireland, see the Wikipedia articles Cromwellian conquest of Ireland and Oliver Cromwell. I found these articles to be objective and balanced. There are of course more polemical accounts to be found on the internet, for example Cromwell, Oliver: Tyranny of 1649 - Irish Cultural Society of the Garden City Area. read more