Trial and error I suppose.
Which part of Celbridge did you move into?, the train may not be an option, if you have to walk to the station.
I live in Celbridge and frequently have to go into town early, and if you are considering going by car, five minutes will make a huge difference.
You could be fine at 6.30 for city centre at 8.00, but 6.35 and you could be 20mins late!
Theres a 67 bus at 6.30, which should have you in with time to spare.