The donegal liner is designed to be attached to your current donegal coat, to make it more versatile and able to take in the depths of winter.
It comes with buttons, but requires you or a local alterations tailor to put in the buttons in the correct place on your existing coat.
The liner is made of the same material as the current coat lining, with wool wadding quilted in between it. Wool is much less common than the synthetic materials usually used for quilting, but is warmer and performs much better with moisture.
The liner also has two patch pockets on the chest, so you don’t lose access to those, just to the larger pocket on the left hip.
The liner only runs through the body of the coat, and only halfway along the shoulder. It is approximately 6mm thick, but unless the coat is already quite tight on you, it should fit under all coats.