Would the EU go to war with the United Kingdom if Ireland is forced to exit the EU? Belfast is 1 example.?

So who would force Ireland to leave the EU and how?  The Republic of Ireland is part of the EU and will remain that way.  Northern Ireland is part of the United Kingdom.  If the United Kingdom leaves the EU, that will include Northern Ireland although there will need to be some kind of cross border arrangement to avoid both sides breaking the Anglo-Irish Agreement.  Northern Ireland is free to leave the United Kingdom and join the Republic of Ireland if that is what the majority of its citizens want.  If the Republic of Ireland wanted to rejoin the United Kingdom, they would get a sympathetic hearing.  The EU is not about war - that's the whole point of it.  Do try to keep up.

No, if necessary a boycott could bring the UK to its knees in a couple of months

EU are imperialists like the UK but they could beat the UK in a war.

The EU doesn't even have an army, how would they go to war?

Belfast, dearie, is in Northern Ireland, which is still officially part of the UK, and UK is supposed to be leaving the EU in a matter of weeks.

The Republic of Ireland, however, is not likely to leave the EU any time soon. The only 'war' will be the war that Britain wages on itself.

War?  No.  They would just call plebiscites until the desired outcome was achieved.  It's a tried and true method for them.

No. AND which Ireland are you referencing? The Republic of Ireland or Ulster (Northern Ireland which is still part of the UK).

I think that is very unlikely to happen, but who knows? wars have already happened for the most futile reasons in europe ...