I have the biography too, which is actually quite vague and doesn't even mention from where he obtained the mirror, and doesn't actually contradict me. In addition, the book was written decades after his death, so the author was clearly unable to ask to the man himself.
The tale I told was a radio interview with Lynott himself which I heard with my own wee ears (Caroline? It was decades ago, obviously). He may, of course, have made it up.