If you're leaving a party, would 'beannacht libh' or 'beannacht agaibh' be correct?

When using "slán" you'd use "le" towards the person leaving.
I don't know about "beannacht" but it may have a similar usage
although the Ó Dónaill and others (dictionaries) don't seem to
distinguish. I might lean towards "Beannacht agaibh".

Wait for more input.

Tim

go mbeannaí Dia dhóibh

"Beannacht Dé [ort/oraibh]!"
The blessing of God on you [all]!

As tiomluasocein noted, it's more customary to use an expression with slán:

Slán leat/libh! (said to the one[s] who are leaving)
Slán agat/agaibh! (said to the one[s] who are remaining)
Slán abhaile! ("Go home safely" - said to the one[s] who are leaving)
Slán go fóill! {"See you later" or "Bye for now")
Slán a bheas tú! ("Farewell")
Go dtuga Dia slán abhaile [thú/sibh]! ("May God take you [all] home safely")
Go bhfága Dia an tsláinte [agat/agaibh]! (response to the above: "God keep you [all] in health")

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.