Is there any trick to replacing the door barrel and key on a TJ magna?
Vision into where it is located is limited as there is no cutout on the inner part of the door shell to gain access.

Yes,

You have to remove the trim, and take out the door handle. You will have access (through a ring spanner and an extended socket) to the 3 M10 nuts holding the handle on.

You will need the old key to turn the lock arm to remove the handle and you will need to disconnect the cables from the locking system.

Mike

"T" handle with 10 mm socket works best, no room for a full size 10 mm socket. Look carefully at how the key barrel is held in the door handle and remove it.
Putting a new lock and using a different key to open the car is a real pain. I did this and always got them mixed up.
Better to take the original key and misbehaving lock barrel to a lock smith and ask if it can be fixed, or swapping the passenger side lock barrel to the driver's side.
If the original barrel is beyond repair, a locksmith can rebuild a new barrel to suit your original key.

another option max is to remove the whole handle and lock , get the same from a wreckers and just get them to swap them over and setup the new lock barrel to work with your key.

had this done yesterday cost me 10bucks from wrreckeers, and 25 from the locksmith

Why are you removing the handle to change over the barrel. Just spin back the circlip and push barrel into the door