The Irish slave trade began when James II sold 30,000 Irish prisoners as slaves to the New World. His Proclamation of 1625 required Irish political prisoners be sent overseas and sold to English settlers in the West Indies.
By the mid 1600s, the Irish were the main slaves sold to Antigua and Montserrat (70% of the total population of Montserrat were Irish slaves at this time).
From 1641 to 1652, over 500,000 Irish were killed by the English and over 300,000 were sold as slaves.
Ireland's population fell from about 1,500,000 to 600,000 in one single decade.
During the 1650s, over 100,000 Irish children between the ages of 10 and 14 were forcibly taken from their parents and sold as slaves in the West Indies, Virginia and New England. Another 52,000 Irish (mostly women and children) were sold to Barbados and Virginia while 30,000 Irish men were sold to the highest bidder.
In 1656, Oliver Cromwell ordered that 2000 Irish children be taken to Jamaica and sold as slaves to English settlers.
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.
The importation of Irish slaves continued well into the eighteenth century, long after the importation of African slaves became the norm. Records state that after the 1798 Irish Rebellion, thousands of Irish slaves were sold to both America and Australia.
Irish slavery didn't end until Britain decided to end slavery in 1839 and stopped transporting slaves.
The enactment of 1652 in the British Isles:
"it may be lawful for two or more justices of the peace within any county, citty or towne, corporate belonging to the commonwealth to from tyme to tyme by warrant cause to be apprehended, seized on and detained all and every person or persons that shall be found begging and vagrant.. in any towne, parish or place to be conveyed into the Port of London, or unto any other port from where such person or persons may be shipped into a forraign collonie or plantation."
The judges of Edinburgh Scotland during the years 1662-1665 ordered the enslavement and shipment to the colonies a large number of rogues and others who made life unpleasant for the British upper class. (Register for the Privy Council of Scotland, third series, vol. 1, p 181, vol. 2, p 101).