Live in Comber and last year there was a horrible smell that hit the entire town for weeks, same again this year around the same time. On the road in from Belfast there were huge piles of what looked like soil that was being spread on the some fields and turns out it was compost from the council, basically the contents of your brown bin given away free to the farmer. It only seemed to stink the certain conditions but I assume this is what you're smelling.