I've always said "plad." But now it's interesting to think about, because my dad had an accent that sounded Welsh to a lot of people, an accent I couldn't exactly hear of course. I'm pretty sure he said "plad" but in a lilting sort of way.
Anyways, "plaids come in various tartans." That is, you have a plaid, which is the blanket garment you wrap around yourself. Tartan is the pattern.
Side story: My whole life, up until a few years ago, I thought this one grandma of mine was a MacFarlane. Turns out she was a MacDonald. My plan is to keep on pretending we're of MacFarlane descent. I'm sorry but "This I'll Defend" is a way better motto than "By Sea and Land," even if it's more suitable all things considered to my family's ways.
Gosh darn it!