Yes it was, as since it was never part of Roman empire it developed from missionary activity, and even started its own monastic missionary activity back to North Umbria, Faeroes and apparently even Iceland.

after non-trivial inquiry from far-away California, my best understanding is that the Celts did gracefully embrace the Christian faith among the monks and those serious about religious life. Since there were vivid and lived religious traditions alive at all times through history, this transition was not uneventful. However the kind of "top down" and by-the-sword conversion that did occur e.g. the Baltic tribes, was not the case with the equally fierce Celts