I didn't even realise there was a Riot in Northern Ireland till I saw this. Apparently it's been on for a couple of days now. Would think the whole world would notice. It took me a while to get to how this riot started but this is what I think happened so far:
Some dudes called 'Orangemen' were on their way from whatever yearly Orange celebrations they're a part of and decided to pass by an area called Ardoyne. Now, this won't really connect until you realise two things:
1-Orangemen celebrate the ass-kicking a guy called William III, Prince of Orange, laid down upon England's King James II and stopped the area from being turned into a Catholic Sweatshop. Now, with that being said,
2- Ardoyne is mainly Catholic and is home to some IRA factions, so you can imagine the balls Orangemen have to just stroll down there every year with their festivities hanging out and all.
Also, looking more into Ardoyne, it seems that the area itself has sustained a shitload of bullet holes and bomb frags in the past- so I'd imagine the people living there are probably made out of thick cement and just hate everything and everyone. Before riots even broke out, there were some articles about how the Belfast Police was going to place some riot control measures around the area. That's probably why it popped off.
Religion is hardcore.

Souces:
-http://washingtonexaminer.com/belfast-cops-gird-for-riot-as-orange-marches-start/article/feed/2011202?custom_click=rss
- http://en.wikipedia.org/wiki/Ardoyne
- http://www.belfasttelegraph.co.uk/news/local-national/northern-ireland/police-officers-injured-in-ardoyne-after-orange-order-march-16184467.html
- http://www.belfasttelegraph.co.uk/news/local-national/northern-ireland/march-tensions-high-at-ardoyne-flashpoint-in-belfast-16184151.html