From what I've read, the earliest references to the great kilt is around 1594. Which just barely puts it in the Elizabethan period (1558–1603); the period usually represented by Renaissance fairs. Before that they wore a 'brat', a wollen cloak, over a long linen tunic. Very early versions might have been plain or a simple checked pattern made from undyed black and white wool.

I've read varying accounts and references across the Intertubes, which of course must be taken with a grain of salt.