African slaves were very expensive (50 Sterling), had to be transported long distances and paid for not only in Africa but in the New World. Irish slaves were cheap (no more than 5 Sterling) and most often were either kidnapped from Ireland, prisoners or forcibly removed.
They could be worked to death, whipped or branded without it being a crime. Many, many times they were beat to death and while the death of an Irish slave was a monetary setback, it was far cheaper than the death of an expensive African. Therefore, African slaves were treated much better in Colonial America