A divided nation would have retained slavery north of the border, too. They just didn't call them slaves -- they called them Irish immigrants, and they weren't treated any better than black slaves in the South.
As bad as things were for the Irish in, say, the coal mines of Pennsylvania, it beat the hell out of their lot in British Ireland, which WAS about as bad as the lot of the black slaves in the Deep South.
Nonsense; people don't emigrate in waves from their homeland to become slaves.