By Sarah Rodman, Globe Staff  |  October 23, 2007

Sinead O'Connor may have kept her eyes on the stage floor for much of last night's performance at the Orpheum Theatre due to stage fright, but her voice remains unafraid.

Twenty years into an unusual but never boring career, the Irish chanteuse remains possessed of an uncommon power and spirit that has not been diminished by the crises, both public and private, that she has endured and sometimes created.

Over the course of a consistent 90-minute show, backed by a four piece band on an unadorned stage, she let her agony, ecstasy, fear, and faith filter through music that went from whisper quiet to storm-force howls.

The sound mix was not her friend at the start - especially the tamped down drums - but the fire of opener "The Emperor's New Clothes" was enough to ignite the audience, as was the wheeling and winding fiddle of its follow-up, "I Am Stretched on Your Grave."

Although O'Connor claimed fear kept her gaze downward, her stage presence did not suffer. She chatted about becoming addicted to television preachers during a brief time living in Atlanta prior to the Celtic-reggae hybrid "Lamb's Book of Life." And she mused about doing interviews with the Christian press for her new album "Theology" and the fact that a small percentage of the interviewers would take issue with her suggestion that "God perhaps doesn't want war."

As a rebuttal she offered up "If You Had a Vineyard," with its direct quotations from Isaiah and lush backing vocals, which pulled the listener along as if caught up in a current.

While "Theology" draws inspiration from scripture, a well O'Connor has tapped since her debut album, "The Lion and the Cobra," it was not the ecclesiastical that produced the night's most rapturous moment. That occurred when O'Connor stood at the microphone and lifted her voice for "In This Heart." As she was joined in harmony one by one by her bassist, fiddler, and guitarist, the intertwined notes pulled a little bit of heaven onto the stage and easily survived a momentary lapse of lyrical memory.

On recent tours, O'Connor shunned her earlier work, but she has embraced it again wholeheartedly. She reached back to "Lion" for the quietly majestic vocal showcase "Never Get Old" and offered six songs from her best-selling 1990 album "I Do Not Want What I Haven't Got," including her hit Prince cover, "Nothing Compares 2 U," and the melancholy yet raucous "The Last Day of Our Acquaintance."