What side does it go on??

[vmac3205]

Men's fly plaids are almost always seen worn on the left shoulder. There are no "rules" about what shoulder to wear it on, like have been invented for lady's sashes.
(See my article on that here).

Sounds like someone is confusing the men's plaid with the lady's sash.

As the fashion of wearing it on the left comes from the fact that most people are right handed, I wouldn't have any qualms about a left-handed person wearing their plaid on the right shoulder -- but I would suspect that most left-handed men wear the plaid on the left anyway in deference to fashion.

Women traditionally wear the sash on the right, unless they are the Clan Chieftess OR are Scottish Country dancers. Then it is worn on the left.

[MacMillan of Rathdown]

Women traditionally wear the sash on the right, unless they are the Clan Chieftess OR are Scottish Country dancers. Then it is worn on the left.

A general note of minor clarification:

I have yet to meet a Lady Clan Chief who uses the style "chieftess", in much the same way as I have yet to meet a woman, ordained in the Episcopal Church, who uses the style of "Priestess".

Ladies who are Chiefs of Clans in their own right are styled "Chief", as in Margret Elliot, Chief of Clan Elliot(t). The same applies to Ladies who are Chieftains of major branches of clans; they are styled "Chieftain". Ladies who are possessed of a feudal barony are styled "Baroness of X", just about the only time that the feminine form of address is used.

In archaic, or extremely formal usage the wife of a Chief, Chieftain, or feudal Baron is styled as "Lady X". The wife of a feudal baron may also styled as "Baroness X" indicating that she is the wife of "Baron X".

In Ireland the more common usage is to style the wife of a chief or chieftain as "Madam" (as in Madam O'Conor, the wife of O'Conor Don). The wives of feudal barons are styled either "Lady" or "Madam" or "Baroness" as custom and preference dictate.