The concept of Irish "slaves" comes up here often enough that it's picked up a section in the FAQ. Others may have more to contribute but the upshot, explained well here is that Irish people captured and put into involuntary servitude were neither slaves nor serfs. They were indentured servants for a period of years. Their condition, though surely unpleasant, was not permanent (unless of course they died before the end of the term) or hereditary like that of slaves and serfs.

Edit--my use if the term "indentured servant" here is a bit sloppy. My understanding upon further review is that, in the usage of the time, an "indentured servant" specifically was one who entered into an agreement to pay a debt through servitude. Cromwell's prisoners would have had a different term applied. That said, the legal status of these servants seems to have been similar once they were in service--the difference being how they got there.