Well, that's weird.
In another Vatican manuscript (Pal. lat. 482) available in digital facsimile from the Heidelberg UB, there is a series of alphabetic signs in an otherwise empty column on f. 15v (click to see the whole leaf on the Heidelberg UB site):
At first glance, this looks like a secret alphabet. In that case, the secondary literature probably starts with Bernhard Bischoff, "Übersicht über die nichtdiplomatischen Geheimschriften des Mittelalters," Mitteilungen des Instituts für österreichische Geschichtsforschung 62 (1954): 1-27. I don't find other examples associating Columbanus with secret writing, but Bishoff notes several attributions of secret writing to Irish clerics.
But several of the letters look quite normal. Is the series rather an initialism, with each letter standing for a word in some devotional passage? If that is the case, the secondary literature one needs is entirely different.
And one can't help but notice that there's a certain symmetry between the haloed "q" and the "p" signs at the beginning and end of the fourth line, or the "Christmas trees" on the left and right side, or the forwards uncial e in the third line and the backwards uncial e in the first line. Was there some kind of mirror-image game at the basis of these characters?
Who knows? It's weird. When you browse through manuscripts, you find weird things.