Redwolf wrote:
One of the things that comes immediately to mind, as it's often said when saying goodbye to someone in Irish, is Slán Abhaile...literally "Safely Home." As it's usually said to someone who is on his way home, however, I'd wait for others to weigh in on whether it could work in the sense you mean.

The Irish actually say "Safe home" to each other in English too, but as Redwolf surmises it usually used when someone is on their way home. Bríd (or Braoin, etc.), would it make any sense to say it before the person left the house?
In some songs about lovers going off to war (such as
Siúil, a Rún), the expression
Go dté tú slán "May you go safely" is used. I think it is taken as a given that if the person makes it through safe they will try to come home or back to their loved ones.
Other songs (such as
Fill, Fill, a Rún, Ó) use the verb
fill "to return". So another option would be:
Fill slán abhaile "Return home safe."
However, this might feel more like a call or prayer to someone who has already left.
Fill slán ar ais "Come back safe" might work too.
To me,
Go dté tú slán "May you go safely" has the best fit for the context, wherever you both are at the time.
Await input from others ...