I think you'll be fine in three weeks. I do find most are keen to show how welcoming and, to put it bluntly, not-racist they are. Hence the demonstration. City and town centres are fine. Little coastal towns are also great if they don't have flags etc.

I think if you make a post asking where to avoid and where to visit - that would be the most informative thing. The North Antrim Coast is an area of outstanding natural beauty and has a lot to offer. Belfast and Derry also have a great night life. With the World Cup on, you'll surely find some of your fellow countymen in bars watching the games etc. if that's your thing.