Sorry that Tabitha is having a rough time right now! I have used mirtazapine on one of my cats in the past and it did work great at kick-starting his appetite, but he had completely stopped eating and the situation was getting serious. I'm not so sure I'd use it on a cat that was still eating just to get them to eat MORE. If it was me, I'd probably try other methods first. Mirtazapine is indeed an antidepressant, for humans (also known as Remeron). In cats, it increases appetite and lessens nausea. Kinda like how cyproheptadine (aka Periactin) is an antihistamine for people but works as an appetite stimulant for cats. Mirtazapine only needs to be given once every 3 days though, whereas cyproheptadine is given twice a day.
There can be some unpleasant side effects with mirtazapine, such as restlessness and something called serotonin syndrome. It is a potent drug, after all. That's why if there are ways to increase food intake without going the chemical route, I'd recommend trying those first. Does Tabitha like any particular treats? Can you crumble them on top of her food? My guys love 100% freeze-dried meat treats like PureBites or Whole Life and if they become picky about a particular food, dusting their meals with some of those gets them eating it again.
Tabitha has renal disease, correct? What is her urine specific gravity, do you know?
If she does have pancreatitis (was an fPLI test done?), she really should be on pain meds (but NOT Metacam!!). That should also help with her appetite.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler