For people living in Englnd, Scotland and Wales, you need to be aware that while DVLA in Swansea processes our driving licences, drivers in Northern Ireland are processed by Driver & Vehicle Agency.
If you haven't seen your diabetes consultant within past year, they may ask you to attend for a medical examination at your GP or hospital clinic, which should be paid for by DVA. assuming you mean a car or motorbike licence. If it's a larger vehicle, whether for leisure eg towing a caravan or horse box, or professionally, then you may have to pay for examination. I'm a bit out of touch, as I only spent 6 months studying and working in NI over 15 years ago, and only had to register my vehicle, as I had to buy a new one, after the one I'd brought from England was stolen and burned out.