Graves built the thirteen sign sequence in The White Goddess, 1948, by pairing his own reading of the Beth-Luis-Nion tree names to a thirteen month lunar calendar he reconstructed himself. That the wider tree calendar is his modern invention rather than an attested ancient system is not in dispute, and the tree names match because he took them from Ogham. What nobody has examined is the step after that: whether placing a given tree at a given calendar position reflects anything in the surviving Ogham material, or only his argument. No Celtic studies scholar has tested his month by month assignments either way, which is why this is an unfilled gap rather than a disagreement between camps. One question covers all thirteen signs, because the pairing is one act rather than thirteen.
What would resolve this A philological comparison of Graves's month-by-month tree assignments against the Ogham letter-name glosses preserved in the Auraicept na nEces and the Briatharogam tracts, by a scholar of medieval Irish, to establish whether any assignment beyond the shared name is supportable.
OpenCeltic studies, medieval Irish philologyDamian McManus, A Guide to Ogam