Fuel Price Discrepancies

So, I've been travelling in a sort of camper in Ireland both North and South and it was all very picturesque and very pleasant. Firstly the population density is low countrywide; In the large conurbations the distance between folks is much the same as the UK and you can be ignored with as much enthusiasm as you can here, but as you leave the towns the people are more friendly, more welcoming and definitely more garrulous. I'm told if you coral rats into ever more confined spaces they eventually start to eat their young; I'm sure Johnathan Swift would have approved. What I can't understand is the fuel prices. My camper is diesel and in the UK I pay the penalty for that by being charged anything up to 5p/l more than petrol for my fuel. In Ireland not only is diesel cheaper than petrol by a significant amount but it works out at about 88p/l. I don't understand this discrepancy since on occasions there was just about 2 or 3 miles in the difference from North to South. Whatever it is, it provides a vibrant business opportunity for petrol stations just south of the border which is only matched by the equally flourishing trade in alcohol in the North from Southerners. Swings and roundabouts indeed