Sorry to hear you are unwell. Are you a member of a union? They would be best to advise you on the process.
All you can do is document your symptoms, the type, frequency, how they impact your quality of life. And if they are improving, getting worse or staying the same. Make a list, be as detailed as you can. Is your gp prescribing any treatment or therapy for the symptoms and has it made any difference.
Think of a typical day, and describe it, what you can or cannot achieve during each day. Can you successfully, dress, cook, clean, shop, garden, relax, travel, eat, sleep, shower etc, independently, with a little help, with a lot of help, or you rely on someone else to do that task.
Occ health is there to protect the HSE in the main, and retiring you on ill health is a cost, so they may wan you to come back to work to see if your symptoms get worse or better.