One thing about Rob Roy that makes me a little bit in disagreement with your comments is the way that Neeson's plaid was during the film.
For example in the Fight Scene he removed the plaid and kept a phillabeg on.
The film is set within the period accepted for the evolution of the Phillabeg in some Quarters.
Rob Roy is depicted as being a man with some means slightly elevated amongst his community. He also wears a jacket in some scenes (albeit slashed leather style).
Therefore my interpretation was that what Neeson was wearing represented a man with a Phillabeg and separate plaid which to me isn't actually a massive leap of fantasy regarding the time period. As I recollect the slightly less elevated members of the cast seemed to be depicted in plaids
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