In the mandala one layer has 9 ogham runes instead of 4. This layer has incorportated the complete Forfeda group of runes. This group of runes has also a different content. Some writeres leave this group out of their describtion. The group has double meanings for 4 of the runes and one has no clear association at all. But the mandala has the Forfeda included, so in order to use the model so must we.
In the mandala tradition the flat figure is a representative of a 3-d model of the universe. Transferring this idea onto Fionn’s window could imply that the stag layer provides a clue to the structure.
The number nine has different meanings in Celtic numerology. One possibility is that the writer took an astrological approach. In the date on which the book has been written the astrological model comprised of 9 layers, earth, 7 planets and the stars. This model would suggest an influence of the universe over the road to the center. This implies a somewhat alchemical influence, in which the emerald tablet states as above, so below. So withou, so within. It is possible the author is influenced, the introduction of alchemy to Latin Europe occurred on 11 February 1144, with the completion of Robert of Chester’s translation of the Arabic Book of the Composition of Alchemy. Well before the book of Ballymote was written.
Another possibility is that the runes in the stag layer do resemble the cosmos, but in a different way. Some author write about the celtic cosmology as if the universe was build of Duiles, which resemble/reflect body parts. Since there is very litle known in writting about the celtic cosmology, this description is personal interpretation. Also if you compare the descriptions to the cosmology of the Germanic/Nordic people. in this myth Odin and his brothers slew Ymir and set about constructing the world from his corpse. They fashioned the oceans from his blood, the soil from his skin and muscles, vegetation from his hair, clouds from his brains, and the sky from his skull. Four dwarves, corresponding to the four cardinal points, held Ymir’s skull aloft above the earth. This would place 9 body parts in the stag layer and 4 dwarves in the ousel layer, which is possible.