The price difference isn't that all that big is it, 1 pound = 1.46 euro + vat @ 13.5% = 1.65 euro. Still about 17% cheaper in NI but they've got to price it at what the local market will accept.
The Tribune at 1.60 sterling would cost, using just the conversion rate + vat, 2.64 euro here, so we're getting a bargain at 2.10. (0% vat rate on newspaper in the UK, 13.5% here)
Possibly people sometimes still do a rough Irish punt to to sterling conversions in their heads when looking at UK prices.