Joseph's brethren famously sold Joseph into slavery:
Genesis 37:28 Then there passed by Midianites merchantmen; and they drew and lifted up Joseph out of the pit, and sold Joseph to the Ishmeelites for twenty pieces of silver: and they brought Joseph into Egypt.
Later Joseph explained that this was intended as evil by the brothers of Joseph, but was intended by God for good.
Genesis 50:20 But as for you, ye thought evil against me; but God meant it unto good, to bring to pass, as it is this day, to save much people alive.
It's not the only time God has used slavery to achieve His ends for the good of a nation.
Around the turn of the 5th century, a man named Maewyn Succat was taken prisoner by Irish raiders: pirates/slavers. God worked in this providence to make of that man a missionary to the Irish nation, leading ultimately to the Christianizing of the island of Ireland.
God's Providence is mysterious. It is said that Mawyn, or Patrick as he came to be called, believed that his enslavement was a punishment for a particular sin that he had committed. Perhaps, in part, that was true. But in hindsight, God's greater purpose in the event was to save Irish souls by the voice of a Welsh preacher.
There are still today many Irish souls in need of salvation, and there are no more Irish pirates to import Christian slaves. In some ways, the Irish condition today is more dangeous now than it was then, for there are many lost who call themselves Christians, whereas then there were few if any. Any modern-day Patrick has an enormous challenge, to shed the light of the gospel in place that thinks it knows what Christianity is, and yet is trusting not in Christ alone for salvation, but in some human system.
While much of Ireland, and much of America as well, celebrates the day with drinking large quantities of green beer, perphaps a sober prayer to God for renewed missionaries to the Irish people is in order. Whether they are brought as slaves (which seems doubtful), or however God chooses to send them, let us be eager that God's flock be gathered unto Him.
May God's will be done on earth, as it is in heaven,