Is "mhaith" lenited because it's an adjective that follows a singular feminine noun?
… a non-genitive singular feminine noun, yes.
is this something you say when you go to bed, or is it a greeting for nighttime?
Going to bed
Does this sound it like. "ee-ah V-ah" or "ee-ah W-ah" I think I hear the mh as a "W" but it might be a dialect thing?
It could be "Ee-ha why" or "Ee-wa" depending on dialect.
Those cover Donegal and Connacht; eehe va in Munster. https://www.teanglann.ie/en/fuaim/Oíche_mhaith