YET ANOTHER REASON FOR BELIEVING ELIWLAD IS A WELSH VERSION OF IRISH AILITHIR

Professor Stefan Zimmer, a noted Celticist, is one of several such language experts who have lent support to the notion that Eliwlad derives via the Welsh from Irish Ailithir: 

"Your proposal to understand Eliwlad as a W 'translation' of Ir Ailithir looks quite attactive. Eli- might well stand for Ir ail(e), and tir is correctly translated as 'gwlad'. The respective range of meaning of both words is, of course, not perfectly identical.

But he added something else as a sort of side note and may, inadvertently, have provided a good explanation for why the Lleu death-eagle motif was borrowed from the MABINOGION by the author of the 'Dialogue of Arthur and the Eagle':

"We have to remind of an alternative, however, viz. that the 'other land' referred to might be the 'Otherworld' , so that the bearer of the epithet may have been named so for assumed/desired magical qualities."

What seems obvious to me is that Ailithir/Eliwlad was interpreted as being a designation for the Otherworld, a land of spirits who could assume the form of animals or birds. This alone would be sufficient to account for the presence of the spectral eagle in the poem.