> I must be missing something here. I deliberately chose 'ingairiu'
> because I thought it was the io stem masculine (or neuter) dative
> singular of 'ingaire' [...] after the preposition 'do'
You're right here. The form "ingairiu" just struck me as odd at
first, but we have good independent support for it in "Táin Bó
Fraích": "oc ingairiu chaírech". I wasn't certain, so I asked.