My reference to Irish slavers has been queried, but others have answered for me. Partularly active in the Dark Ages, of course. And, since this is a folk forum, surely the most dramatic account of slaving in song is "The Flying Cloud"
"My name is Arthur Hollandin
As you may understand
I was born and raised in Dublin town
Down by the salt sea strand"

and the extraordinary verse:
The plague it came and fever too
It killed them off like flies
We piled their bodies on the deck
And hove them o'er the side
For sure the dead were lucky then
They'd have to weep no more
Nor drag the chain nor feel the lash
In Cuba for ever more"