Dear Sir,

I am interested in knowing from your organization if there is a traditional
Irish blessing for a bride and bridegroom. My sister has the good fortune
of marrying an Irishman later this month. There will be many guests from
Ireland who can speak Gaelic. I felt that a nice gesture would be to
recite a traditional blessing.

Any assistance you could give would be much appreciated.

The phrase "Go maire sibh bhur saol úr" would be traditional and the response from the couple would be "Go maire tú do shláinte."

(Rough translation: May you live/enjoy your new life / May your new life be long and the response, May you always have good health/May you live in health. It's very awkward to translate).