It's her made-up language, mostly--what comes through from her fuller self, though it probably has some Gaelic roots, even though she's Australian! You might enjoy the film Toward the Within. It's got both the music (of Dead Can Dance)and the group members speaking about the process. She's an astonishingly soft-spoken and delicate-looking person, and when she starts singing she inhabits this much bigger 'place'.
I knew there was more to this language of Lisa's;now I remember her talking about it a bit:
Glossolalia (from Greek glossa γλῶσσα "tongue, language" and λαλεῖν "to talk") is the vocalizing of fluent speech-like but unintelligible utterances, often as part of religious practice. Its use (including use in this article) sometimes also embraces Xenoglossy - speaking in a natural language that was previously unknown to and that is not understood by the speaker.