The fact the RT is a premium product with no competition in its price range seems to be justification for the large price increases that keep happening every year.
The Radio Times does have competition, though - the Guardian Guide, the Sunday Times Culture section, the Mail weekend magazine and all the other TV guides in the broadsheets and mid-market papers. It also used to have Time Out before they stopped doing TV listings. That's their competition, so there's plenty of other places for people to go if they don't like it. They don't just put the price up on a whim, either, they're a commercial product and they charge what they think people will pay for it. It's not like the eighties when you were forced to buy it.
Also, how long as the Radio Times included RTÉ for NI?
Well, this is going to make me sound like an enormous bellend but in the mid-nineties I did actually aim to buy a copy of the Radio Times for every region in Britain - back in the days when they did several editions and they were different enough to be a bit interesting (to me, anyway, not to anyone else). So every time I went to a different region I'd buy one. Within two years I'd got an issue from every region apart from East Anglia, but by that point girls and pop music seemed to be a bigger priority than arranging a jaunt to Norwich. I've chucked them out since (though my main Radio Times collection remains very much intact).
What all this means is that I did have an Irish edition, bought on a daytrip to Dublin. As Tony Currie's book points out, after deregulation in 1991, they rebranded the Northern Ireland edition as Ireland and started including RTE listings - in the space other regions used for regional variations, though in the same larger font that the Scottish edition used for Grampian and the Welsh one for S4C. It livened it up and it meant they could sell it in a wider area. And seemingly they've never stopped. You can see both on both sides of the border, so why not?