The English equivalent is Molly, which is usually used as a name in its own right now, but used to be short for Margaret, or more rarely, Mary.
My Scottish grandmother used to call my mother Mhaili although my mother is English and her name is Mary. She used to roll it off her tongue so the L was pronounced in between an English R and L sound, and it sounded quite pleasant. The H was almost completely silent although there was a very slight breathy tone to it.
I believe it is pronounced Vaalee and is a form of the name Maili. The English version is Molly. Mhaili is not a name in itsel but in Gaelic, it shows that someone called Maili is being addressed.
Gaelic nouns have cases just like in Latin. Nouns beginning with M become Mh (pronounced 'V') when in the vocative case. So if you are talking about someone you will refer to them as Maili. If you are talking to them, however, you will call them Mhaili. In Lowland Scots, the distinction is frequently lost and people will be christened with one or other form of the name. This can cause confusion. As a boy I was always told that Seamus was the Irish for James and Hamish was the Scots Gaelic form. In fact Seamas ( there are a few variant spellings) is the nominative form and Sheamais (pronounced Hamish) is the vocative in both languages.