Again, it seems, I'm with JennieG.

In Oz I've only heard the word "plaidie" rarely but it was always "PLAY dee" whereas I've heard the word "plaid" routinely, always pronounced "PLAD". Because daughter #2 has taken up Scottish Highland dancing I get to hear the words plaid and tartan quite often, frequently by speakers with a lot more recent Scottish (and Scots) in their lives than mine.

Cheers, Rowan