Thanks for the context - "Beannacht Dé" would be entirely appropriate here. Make sure that you put the accent on the E - if you don't, the whole sentence makes no sense.
Rather than Oíche mhaith. Beannacht Dé ort, I would opt for Codladh sámh agus beannacht Dé.
That means sleep well and God bless. You can add "ort" at the end of beannacht Dé if you are sure nobody else from the family will be buried in the same plot. If that isn't the case, don't add "ort" since it refers to one person and will therefore have to be changed if anyone else is buried there at a later date. Without "ort" it means "God bless" for any and all who are buried there.
Definitely don't trust Google translate for gravestones - it is completely wrong. Dia duit as mentioned above, is the way we say "hello" in Irish. It would make no sense at all in your context.
You could use the more traditional sayings as l'Irlandais said. These include Ar dheis Dé go raibh a hanam (may her soul be on the right hand of God), or Suaimhneas síoraí dá hanam (may her soul have eternal peace) but with these two they refer to one woman only, so again if anyone else is to buried there in future, or is already buried there, I would avoid these.
Anyway, if you want the personal touch, then you don't need to go with the set phrases and you can just go with Codladh sámh agus beannacht Dé - it's your own personal message that way.