The 7th to the 9th Century was a kind of "golden age" (if any age is) of Celtic Christianity. The church of Rome before this had not evangelized northern and even central europe--the first areas to sucumb in the collape of the Roman Empire.
It took independent Celtic missionaries, based in Ireland and Scotland, to bring Christianity to Germany and much of France...even down into northern Italy south of the Alps. Most of the tribes in this area were either out and out pagan, (worshiping demons and inanimate objects...animism) or Arian, a cult honored Christ but which denied His divinity.
This is covered, I am told, in the book "How the Irish Saved Civilization." Interestingly, many historians believe St. Patrick, of the 5th Century, may have never visited Rome, nor, they think, was he interested in support of the papacy. (ie. Patrick may not have been in fact, Roman Catholic) Don't tell the Irish Catholics though!
In the late 7th into the 8th Century (starting with an agreement in England in 664 at Whitby Abbey) the Celtic churches came under the authority of the strengthening Roman church, and Celtic missionary activity came to an end, as the Roman brand of catholicism moved into control of Christianity in northern Europe.