> You say this word is late, but I find it listed in various Old Irish
> word lists. Is its inclusion in such lists an error?
It would depend on the purpose for which the list was compiled. If the
list was intended to be a list of words attested for the Old Irish
period, then yes.
You can see the attestations in DIL for yourself; they are all Modern
Irish. DIL cites the word from Keating's 'Three Shafts of Death' (17th
century), 'Duanaire Dháibhidh uí Bhruadair' (17th century), 'Dánta
Aodhagáin uí Rathaile' (17th-18th centuries), and 'Trompa na
bhFlaitheas' (18th century).