> I can see some similarities between 'as-bert' (says) in Old Irish and
> the modern verb 'Abair.' But I don't see a 'd' in any of the old
> forms and I was wondering how a 'd' got into the modern language in
> words such as 'dúirt' and 'deir'.
The "d" is the reflex of an infixed 3rd sg. neuter pronoun that became petrified in Middle Irish. The development goes
1. as·beir "he says"
2. at·beir /ad·v'er'/ "he says it"
3. the pronoun becaomes petrified and loses its meaning, at·beir is now used for simple "he says"
4. the lenited "b" after /d/ is lost; the unstressed initial vowel before the /d/ is likewise lost: 't·(b)eir --> /d'er'/ = deir
The same in the preterite:
5. The lenited "b", when still present, coloured the following vowel so that it became "ú". The palatalised "rt" is due to
various analogical restructurings --> 't·(b)úirt --> /du:r't'/ = dúirt