What if Columbanus had never left Ireland and never created monasteries in Europe?
Well, the obvious answer would be "someone else would have", but I think this question deserves a bit more developed answer.
Monachism in Europe wasn't unknown at this point, far from it : Saint Cesaire Saint Honorat, among others, created monastic rules in the Vth century in southern Gaul which had some important success (and influed on St Benedict rule) while other rules (while not monastic per se) existed for clerks before. These rules were widespread but not systematical at all, and in practice each monastery obeyed its own customary rule. The lack of strict unity (altough each particular rule could be tied and generally similar to others) and the dependency on local/regional powers led western monasteries to be, while important religious centers regionally, not as pregnant on the society they were.
Basically, rules tended to be tempered, monasteries tended to be too tied to political power; not unlike it happened in the Xth and XIth before the Clunisiac revolution.
When Colombus came in Gaul, his monastic practices had big advantages for both the Church and the King : first, it was an extremely demanding rule which really contrasted with the relatively easygoing practices of monasteries, then it stressed a direct dependence from Rome and from the royal power, and eventually it kept a political and institutional independence that pleased many people from both lower to higher classes (while displeasing some, such as Brunhilde).
Eventually, the Hibernic practices were mixed with continental institutions (the primordial role of bishops was never put in question), and revivified Latin monachism trough a later mix with Benedictine rule.
So what could have happened without Colmbus? Not the absence of monachism, that is certain. But the absence of a certain kind of triumphant monachism, maybe.
Let's keep in mind, that while Ireland was a vibrant and dynamic religious center at this point. Maybe that someone without the dynamism and strong spine of Colombus would have replaced him in Francia. While I doubt it would leave monachism in the state it was in the VIth century, maybe the lesser prestige of continental monasteries would have led instead to an inner change and clerical revolution, possibly led by local monachism in need of religious and political independence and clarity (not unlike Clunisiac reaction to the mess of Xth century, again). It's quite possible that this monachism would be royally supported by Merovingians, as benedictine rule will be by Carolingians.
Meaning a situation superficially similar to IOTL, while with a more relaxed rule and without "need" of a Benedictine reintroduction to temper Columine rule. Superficial because you'd have more of a regional rule situation IMO, as long as the king remains the effective head of his church (as it was the case IOTL), and that political turmoil may represent an hinderance for monasteries, more than it was in the fall of Carolingia.
If you're interested, may I suggest you
Before France and Germany: The Creation and Transformation of the Merovingian World, by Patrick J. Geary?