Everything you say is absolutely true, Lincoln was no saint, and poor Irish Union conscripts did not fight to free black people. Lincoln even let Union slave states keep their slaves for awhile which was both brilliant politically and more than a little cynical.
But in the end of the day Lincoln freed the slaves. History looks at results, not character, methods, or intentions. In the end, Lincoln freed a race of people from bondage and put an end to an unambiguously savage and evil institution. Because of this, everything he and his army did is justified.
The victors do write the history books, understandably, so it's not that easy to find the truth about how blacks were treated in the Northern states. But it's out there.
Results, and not character nor intentions, you say? I've given them to you----the factual, terrible effects of Northern racism on the lives of black people. You can find even more at Kackikat's link, post 112.
Some might say that the situation in the North was a type of bondage all its own.
There is a reason that Lincoln is referred as the Great Emancipator.