Cut the air miles. Don't consume any food that you know has been imported by plane (apart from Fairtrade products).
This one was surprisingly trickly, mainly because most food doesn't say where it came from, or how it got to my local shops. The main exceptions are meat and vegetables, but I was able to get locally produced stuff there.
Assumming that all food produced in the UK is brought to Belfast by boat, rather than plane (which I think it is, by and large) made most things a bit easier to come by, but in the end I was able to source all my food for the day from producers and companies in Northern Ireland, without too many dietary adjustments.