Status Updates From Between the Realms

In the chapter on the concept of an Otherworld, the author states that there are many different otherworldly realms in Celtic myth. But in addition to that, there is an under/middle/upper world division. This last statement is presented without references to sources. I've seen this claim thrown around a lot, but sometimes I wonder if it's not just an attempt to reconcile core shamanism with what we know of the Celts.