How come no matter how many times I select the UK and Ireland section, I end up with US only questions?

There's a UK & Ireland site for Yahoo Answers but the problem is that there is no filter any longer.

Previously if you used the UK & Ireland site you could filter just on questions posted via the UK & Ireland site or all English questions. That filter has been removed and so there is no way to choose any longer. As a result all the English language sites are essentially the same.

All you can do is to post the fact that you are missing this feature by adding a vote to one of the current threads about it e.g. https://yahoo.uservoice.com/forums/211240-us-answe... and/or creating a new one, and hope that staff take the feedback onboard.

There is no UK/Ireland sexction anymore, only the English language section.

You don't. Every English language section now shows all questions in that language. It appears to be mainly US questions, because there are a lot more users there than anywhere else.