Tell your DH to sod off, he’s being incredibly mean not going with you in the first place to celebrate his fil birthday. He can just about get away with it if he doesn’t take the grandchildren with him.
HIBU. Not just because you've already booked it, but because it's their granddad's birthday. That's more important for them to be at than his friend's wedding. If the kids want to learn about Vikings, take them to York sometime.
Your husband is clearly wrong. Grandads rare big birthday party is clearly more important than wedding of guy DS1 doesn't even know.
Your husband needs to explain to DS that the wedding weekend sadly won't be a good time to learn about vikings because he'll be too busy doing boring wedding stuff, and make it up him with a Viking-related trip some other time.
I think DH should have declined the wedding. You already had plans. FIL trumps flaky friend. He won’t get to see much of friend at the wedding anyway most likely. Extra unreasonable to woo DS to change his mind. I’d say DS1 can go but sacrifice next birthday/Christmas present. Or maybe not. Perhaps now is a good time in honouring your commitments.
I would have been annoyed at my husband pulling out to go to the wedding in the first place to be totally honest but you mention having some space isn’t a bad thing so fair play if you’re fine with that. But to then try and wangle it so your son joins him too! I would be completely livid. It sounds like you have a lovely dad/grandfather to your kids who will probably be upset if all of his grandchildren aren’t there. Let alone the fact you’ve booked and paid for it all! I would 100% stick to your guns and enjoy it
You should have taken dh into another room as soon as he brought it up. Then he should have been the one to go back in and explain why the boys are going to Ireland and apologise to you and him for suggesting it. It's totally out of order.