"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

[JemCiasteczka]

When doing the Rosetta Stone Irish course, they taught me, if I remember correctly, Oíche mhaith agat for good night. Does this work too?

[deactivated user]

óiche mhaith agat is probably an attempt at a literal translation of "have a good night", or perhaps an elided version of the more grammatically appropriate subjunctive form go raibh oíche mhaith agat. It crops up occasionally in some older texts, but is relatively uncommon, compared to just plain oíche mhaith.

I think it would sound a bit artificial to most people nowadays, but that's Rosetta Stone for you.

[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