You have this the wrong way around. The Good Friday agreement made close cooperation and 'free trade" (of goods and people) between Ireland and Northern Ireland possible/desirable and when the UK left EU this became impossible if there were a border again between North and South. There either had to be a border in or at Irish Sea or in Ireland. Having a border 'within the UK' - in Irish sea - upsets (some) Unionists, having one in Ireland upsets many people on both sides of the community and is against the Good Friday Agreement. Those rioting are, mostly, protestant unionists who have suddenly realised that by supporting Boris they have shot themselves in the foot. Interestingly, a majority of people in Northern Ireland voted to remain in EU and many people warned Boris that his decision to have a Hard Brexit was going to result in exactly what happened.
This from today's Guardian gives some background,
https://www.theguardian.com/uk-news...ls-for-summit-on-violence-in-northern-ireland