Why buy a slave and have to be responsible for feeding, clothing, medical, care, and housing, and always at risk of losing that investment to misfortune when you could hire an Irishman for the more hazardous jobs and be out nothing but a pittance to the widow if the worst happened?
When tobacco hogsheads and other heavy cargo were loaded in sailing ships at the wharf, it was the Irish who were in the hold securing the cargo, not the slaves. Criminy! the slaves were far too expensive for that duty.
Manumission would have come in time, without any war at all. Northerners just used the abolition movement to foment unrest to justify other nefarious policies against the South.