For a while there, Sainsbury's were dear as ****. Although they have reduced their prices recently. Nobody matches ASDA's fuel prices though. Got diesel in Ards yesterday for 113.9. I think UL was 111.9
Go in Cookstown is cheapest in N. Ireland - 109.9 last weekend. Always 1p cheaper than ASDA. Bit far from Belfast, but handy enough for me going to the OH's parent's house in Dungannon.
Hopefully the new Go garage they're building up in Carrickfergus will have the same prices and set a trend for others. Still too far away for me though...
Although unless you're passing a cheap station, or are within 5 miles or so. It's still be cheaper just to stay local unless there is a huge difference in price.