you got it, more oxidation if there is headspace. topping up with the irish red was a good call. small barrells are already going to mico-oxidize quicker than big barrels because the o2 is coming through the wood and surface to volume ratio and all that.
I got a bunch of those little glass button things that go in planters and boiled the beejeezus out of them to sterilized ( I think I even used a presure cooker) then added to the barrel till it was right up to the bung.
that's right, I filled it to the bung with glass buttons