I am travelling with 2 passports: one UK, one  Irish. My current Thai Retirement visa is in my UK passport but that has no pages left. The Irish passport is still empty of stamps. To avoid having to apply for a replacement UK passport, will Thai immigration allow me to present both passports to airport immigration. They can check the validity of my visa in my UK passport but  start to fill the space on the Irish passport with the stamps? Both passports have a another 5-years to run. Thank you for any advice 

I think your better being safe than sorry and get a replacement British with more pages .IMO I  Can't see them being able to check your retirement visa validity on your British and transfer it to your Irish even though u have dual citizenship ,

A friend tried exiting on a British PP  to Cambodia and entering on an Irish because his British was packed with stamps and was refused entry but this was a 30 day free visa.  

Maybe someone here was in the same situation and can give better advise but personally I wouldn't take the chance . 

Good luck  

They will not do it.

Even if the passports were from the same country they would not use the visa in a valid passport and stamp you in into the country in another a valid passport.

Do you have a visa from an embassy or consulate or an extension of stay you applied for at an immigration office?

You will need to get a new UK passport.