In general prepaid subscribers may do better here, as they can use credit throughout Ireland without having their minutes confined to three quarters of the country. For bill pay O2 actually do have a provision to use minutes in NI, they tried to abolish this in September, but there were so many complaints that they didn't. That said O2 are indeed decreasingly helpful, one good resource is their forum. You can ask the forum people to put you on the tariff. See this thread on the cross border issue
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