Your seller may have accidentally ticked N. Ireland as a place they do not wish to post to.
Ask them to check their Site Preferences untick that box, perhaps put you on their buyer exemption list ,too. Then, you should be able to pay for the item.
Seems they have sold to N.I. before remind them that Royal Mail charge the same as in any other place in the Country, but that Evri and other couriers are more expensive.
@toymaster1jason