More Proof that the Form Μαρκωσίων is Unusual

I can't for the life of me see why Irenaeus retained the ος ending on Μάρκος to make Μαρκωσίων, Μαρκώσιοι (= those of Mark). I think Greeks would have found this form very unusual. The natural formation at the time would have been Μαρκίων - i.e. Μάρκος -ος + ίων. I don't think Greeks would have understood right away that Μαρκωσίων was connected with the name Μάρκος. In tenth century Greek for instance we see it translate the

After this, again, three marquises (μαρκήσιοι) came from Burgundy to Papia with intent to expel its possessors and possess it themselves (Constantine VII Porphyrogenitus, De administrando imperio p. 111)

The French time 'marquis' is μαρκήσιος in Greek. The plural here is formed quite logically (μαρκήσιος -ος +οι = μαρκήσιοι.