Yes, it is possible. Her employer has to be convinced that the illness is sufficiently serious and permanent, and that is a medical matter. She should advise her employer that she wishes to take early retirement on health grounds, and they will go through the steps with her and her medical advisers.
In general, public service pension schemes have special provision for early retirement on health grounds. There could be a top-up to her pension of about 5 years, meaning that her pension could be 25/80ths of her pay level at retirement date (made slightly more complicated by PS pay cuts and the Croke Park deal, but overall the deal she should get is fairly good).