If she had a prom dress or formal dress for the end of secondary, that will do very nicely for a ball. Some of them are just glorified discos really. There are a lot of ceilidhs though, as the first part of a ball or as an event in themselves, so she might like to make sure she has a swingy skirt or dress (or at least one that isn't bodycon) and a pair of flattish, comfy shoes (or even trainers) for those. I got a tartan skirt from one of the charity shops for that purpose!
Thermal layers, hiking boots, decent waterproof for the field trips are a good idea. I would say absolutely prioritise the boots, and then good socks, and good gloves! I skimped on the gloves at first and regretted it. One good pair of each should have her sorted.
For in town, get a coat with a hood or a wooly hat. Don't bother fighting a brolly in the constant wind.
If she wants a gown, they have a practical if unpublished use as an extra blanket on top of the bed!
If she's planning to come home for reading week she will know better what extra bits she wants by then.
Congratultions to her, hope she has the time of her life. It's a wonderful place.