I'm afraid I can't claim any credit for that either, it's a Bachmann model!
To answer your question, I should think the K1s would've encountered 6 wheel carriages at some point in their lives, although I highly doubt 388 in this Rosslare boat train livery would have done. I've seen photos of K1s on older bogie stock, perhaps akin to the Rapido Evolution coaches:
I bought the Genesis brake 3rds for two reasons. One of which is using in short mixed trains, the sort of thing I'd enjoy running behind a tank engine with a few wagons. The other is to support Irish RTR, as the more popular steam-era Irish stock is, the more likely manufacturers will see there is money to be made in making models of the locomotives, wagons and coaches we like.