What does mirelaigue a French patois word mean in English?

Mirelaigue doesn't seem so much patois than an outdated Word. It is most likely a portmanteau Word made up of "mire" (from latin "see") and "aigue", the old French form for "water". As a placename, expect Mirelaigue to be on a high ground, where you have a large and direct view of a body of water.