Here's a report on Marching Day in Northern Ireland.
The Protestant Orange Order is able to muster a lot of people, including a 6-mile long parade, ostensibly to celebrate a battle 330+ years ago. I write "ostensibly" because it's really an assertion of community identity, at least incidentally in opposition to their Catholic neighbors.
Compare that to the remembrance ceremonies of the white South, celebrating the Confederacy of 158 years ago. I'm sure there are some scattered around, but they aren't significant enough to warrant much attention. Why the difference?
You suggest one is celebrating a victory, the other an ultimate defeat?
That might logically make a difference, but where are the big parades celebrating the Union victory? The closest we can come is the Juneteenth observances of recent years. And, more importantly, there's no organization dedicated to the celebration, as well as agitating for the cause now. We had one such organization, the Grand Army of the Republic, but the GAR ended with the last vet, in 1956.
So why do Americans forget the past more easily than those in Northern Ireland?
I suspect part of the answer is immigration: we've added millions of people who've no live interest in the fight for the union or the abolition of slavery.