Metzy's Taqueria. 'Fresh. Food. Fast.' New kid on the block, serving a nice selection of Mexican soups, tacos, burritos and drinks. Location varies, but they're usually at the airport on weekends. We've only tried a couple of things, and they were pretty good. The lines are often long, though, so they're clearly keeping people full and happy. They also cater, and deliver (with a minimum order), and you can order online (sometimes) and set a pickup schedule.
They move around Newburyport from time to time, but their website is kept fairly up-to-date with hours and locations, so check it out beforehand or just swing in if you're on the way to the Island and the line isn't too long.