The easy answer is that they believed in mission and did go out and talked about God..

But that is probably not the answer you are after. Western christianity formed and was culturally relevant to the postlatin world. But north of europe was never part of the roman empire and they did not share all the cultural references.

In Ireland a monastic movment begun with St:Patrick during the 5th century (note that it´s been 100 years of christian statechurch in the Roman empire, the church there has in many ways stoped in it´s development). The irish movment did become culturaly relevant in a celtic culture, a thing that nearly all succesfull missionary movments have. The movment also had mission as a big part of it´s movment so they did continue to spread christianity.

The monks did begun to leave Ireland to Scotland/England and continued to found monastaries that did spread christianity. From these places visionaris did come that walked out to europe to revitalise the church in France and to spread christianity in northen europe.

And u/NFB42 is totaly correct that one of the culturally relevant things was that they didn´t focus on cities, instead towns formed around the irish christian centers.

From what I was taught recently, and if I'm wrong I'd love to be corrected:

In Ireland there developed a tradition focussed on rural areas, and evangelising such areas first. In the traditional 'Roman' areas the norm was to focus on the cities and evangelising outward from there.

Since in the early medieval period there weren't many cities outside of Roman Europe, there wasn't as much evangelising from the traditional, Roman, heartland. Instead it were the Irish who'd go out to the rural areas of 'barbarian' Europe and found monasteries and spread the faith from there.

Again, this might be wrong, but if so I'd love to be corrected too!