Stately, plump Buck Mulligan came from the stairhead,
bearing a bowl of lather on which a mirror and a razor lay
crossed. A yellow dressinggown, ungirdled, was sustained
gently behind him on the mild morning air. He held the
bowl aloft and intoned:
—Introibo ad altare Dei.
How in the hell am i supposed to comprehend this?
People will help you, though this passage is pretty straightforward: Buck Mulligan has his shaving things and is imitating/mocking the ritual of a Catholic mass.