South of Dallas in Texas, along Highway 45 between Dallas and Houston, there's also a town named Ennis. Not far from Ennis, TX is a town (village?) named Alma. They're both in Ellis county. Here's a map of Ellis county where you can see both, Ennis and Alma:
(look at the right lower end of the map for Highway 45. Follow it and it leads you first to Alma, then to Ennis)
Naturally, I have no idea where Annie Proulx first encountered the name Ennis. But I firmly believe that she meant Ennis Del Mar to be an island in the sea.