They have a point. You didn't see ROI going to London to make NI have the same VAT as ROI when many people were going up North to shop. Even though they weren't happy about the VAT rate.
The reason that the ROI government did not lobby London was because no government has any right to interfere with the internal poilitical decisions of another country...pure and simple. Let's not pretend this was some charitable decision on the part of Dail Eireann.
Time to get real here folks. NI people can rightfully lobby for betterment of our own economy, just as ROI folk can for theirs. In this case, NI is arguing for the equalising of a tax that advantages our nearest competitor...we are lobbying our own government, about an internal tax. That's exactly what our representatives are paid to do.