"Good night."

Translation: Oíche mhaith.

[Bryan.EDU]

Is "mhaith" lenited because it's an adjective that follows a singular feminine noun?

[scilling]

… a non-genitive singular feminine noun, yes.

[fireflower201]

is this something you say when you go to bed, or is it a greeting for nighttime?

[Pol-Cowzer]

Going to bed

[Jb11131999]

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?

[vesperlunalobos]

It could be "Ee-ha why" or "Ee-wa" depending on dialect.
http://www.focloir.ie/en/dictionary/ei/Good+night

[torowan]

Those cover Donegal and Connacht; eehe va in Munster. https://www.teanglann.ie/en/fuaim/Oíche_mhaith