I believe that a historical agreement with the Irish Republic going back to independence allows Irish citizens to join the British forces without a British passport.
impartial
You have made some very inaccurate assumptions about the RAF:
There is no difference in the medical employment standards required of fast-jet, transport, helicopter or other pilots in the RAF. There are many pilots in all types, whose eyesight deteriorated after they had joined the RAF, who now fly with glasses. I was one of them.
There is no significant difference in the cost of training a Tornado F3 pilot and, say, a C17 pilot.
The RAF does not own or operate any 'unpressurised fighters', other than the Spitfires and Hurricane of the Battle of Britain Flight.
You cannot choose which type of aircraft you go to, nor which general field you are employed in, although you may be asked what your preference would be. They will take little notice of your answer!