No a journey starting outside the UK but via the UK is not duty payable .
Of cause you are correct in that your flight with Ryanair wozit or who ever from the UK to Dublin is paid at short haul rates not disputed , however the layover period is a catch all it’s there in black and white from several readings and it states that a layover period between flights should be 24 hours if not to be subject to the long haul duty levels for the entire journey.
Now I don’t think anyone has been proceuted its pretty hard to chase a tax liability on a flight outside of the UK isn’t it .
imho the law and regulation is really a mess or diliberately poorly written. After all it was partly introduced as green tax and it’s certianly destroyed competitive domestic air travel.
And the rates are now at extautionate levels without doubt.