Well, to answer about the long a sound. The name Mairi is properly spelled Màiri. The accented a indicates that the sound is held twice as long as without. That's why the sound is Mah-ree. As HughM explained, this is the actual name of the person. When addressing them, we use the Vocative. This is equivalent to the English "O Mary", we say "A Mhàiri". This is known as aspiration or currently as lenition. The "h" is there to indicate a change in the sound. Instead of the M sound, with the h there, we get a V sound. IE: A Mhàiri would sound as if you said Ah Vair-ee.