Liz wrote:

> 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 
like this:

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:

1. as·bert
2. at·bert
4. 't·(b)ert
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