"Are you in Shetland, Margaret?"
Translation:A bheil thu ann an Sealtainn, a Mhairead?
It is a leniting vocative particle that is always placed before a consonant in vocative case (but not in nominative). It is explained in the tips and notes for Phrases and Names skills. The tips and notes are available in the web browser version of Duolingo (but not in the mobile app). You can also access them on the duome.eu website.