I'm not sure what "late medieval" illustrations you are talking about, but the feilidh-mhor cannot be documented any earlier than the end of the sixteenth century (1594) with the first illustrations not coming until early in the seventeenth century -- and most of the illustrations I have seen show it worn with the upper part secured in some way on or around the shoulders, so the kilt shows only knee length. Just from my own memory, the only illustrations I can think of that show it with the upper part down are eighteenth century. So hardly "late medieval" by any standard, so I don't think what you are describing is a feilidh-mhor.

My best guess would be that you are thinking of the leine, or Gaelic tunic. Read my article here:
http://albanach.org/leine.html

You can see some illustrations of this garment. It was worn full length by women, and some men of high station. Most men, especially those who engaged in a lot of phyisical activity like soldiers, farmers, etc., wore the leine to the knee only, either by hiking up a full length leine giving the upper part a blousier appearance, or simply by wearing a leine cut to the knee.

See if any of the illustrations in my article are like those you are thinking of. In any case, this is not a kilt at all by any definition, but a long shirt.

Aye,
Matt