It's great that you are planning to visit Glendalough
! The monastic site and the entire valley are definitely worth the trip to the Wicklow Mountains
. Will you be staying in Dublin the rest of the time?
While I don't know French tour operators organizing this tour, I'd suggest you reach out to French-speaking tour guides in the Dublin or Wicklow area. These can be found on the Irish Tour Guides
Otherwise, it's also possible to take St. Kevin's Bus
and see the valley yourself.