I imagine they were officially described as "indentured servants." The English Régime always likes to cover its crimes against humanity with such euphemisms. But of course it did happen and there is plenty of documentation of it happening. In fact, the slavers got so greedy that they even started kidnapping the children of Protestants in Ireland and shipping them to the Americas. The protests against that are well documented.
According to the book, 'White Cargo, by Walsh and Jordan, there was controversy when some of the "Old English" were sent as slaves. Although the "Old English" were Catholics, they were not intended to be enslaved. It was the Gaelic Irish who were intended to be sent to the Caribbean. So there was a racial basis for the enslavement.