Well, yes, I suppose we could analyse Athirne as athair-ne, but I can't quite see how the meaning would fit the character.
Also, it is surely more difficult for a vowel to be spuriously inserted (Athirne going to Aithirne) than for a vowel to be dropped (Aitherne going to Athirne). For example Aithechdae occasionally became Athechda.
So, on balance I would take the name as Aith-ir-ne, the character having the qualities of the divine ancestor Ír!
> Sent: Sunday, January 06, 2019 at 9:35 AM
> From: "David Stifter" <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re: [OLD-IRISH-L] The Athirne and Midir incident
> From: Old-Irish-L <[log in to unmask]> On Behalf Of Martin Counihan
> > I should think that the spelling should ideally be Aithirne, the first element being the prefix aith- (see eDIL under "aith").
> Is it? It could also be a derivative of the word for "father", i.e. corresponding to a formation like Lat. paternius. Aithirne would be a normalised spelling, in any case. In Old Irish practice, Athirne is a legitimate spelling.