How long do you plan on leaving it in the barrel? If for an extended period of time, I would advise against dry hopping in the barrel, as you can pull grassy flavors from prolonged exposure (> two weeks). I would barrel age first, then either:
1)Transfer to keg, dry hop in a sack, purge with co2, dry hop for X amount of days, pull sack, carb, then serve.
2)If you are bottling, then dry hop X amount of days prior to racking to bottling bucket and bottling.