.....yes, a handmaiden to dionysos the greek god of wine and weirdness. also, for the romans the god named bacchus, that's where the name of baccanalian rituals come from. so far, we know that.
but what wasn't mentioned in the show or here is, was does a maenad.
she feds on feelings. prefering all feelings that are coming from the source of her god. means beeing drunken, lust, sexual tension, voilence and bloodlust. that's why these orgy's are going so far. the more people are going into that stream, the more a maened can fed of it. It's the secret of her immortal life. As long as she can feds, as long she will have her powers. It's also about nature. maenad's need woods, a lot of nature around. If there would be woods in afghanistan or irak, i bet a meanad would be there because of all the violent tension.
orginaly maenad's where the priest's of dionysos, the celebrate extatic orgy's, drove themeselve into a frenzy, where, as greek literature wants to know it, they ripped bate's (mostly animals) apart with there bare hands. and they drove themselves into such a madness, that they ended their own life's as a sacrifice for their god.
so i hope i could give you a better understanding of what maryann is. just knowing that she's a handmaiden of the god dionysos is a little bit less. except someone studies greek mythologie.
sorry for my english, it's not my mothertongue.