>> Téit Dick. = Dick goes.
>> Ní·tét Jane. = Jane does not go.
>> Do·tét Sally. = Sally comes.
>> Ní·táet Dick. = Dick does not come.
> Could there have been an infixed object pronoun in 'Do·tét' a long
> time ago, so that it meant literally 'goes to it' or 'goes to him'?
I've never heard mention of that. It is, I believe, fairly
uncommon for an infixed pronoun to function as an indirect
object (to him, etc.).
> 2. Since the preposition 'do'causes lenition I would expect the 't' in
> 'Do·tét' to sound similar to English 'h'. That may explain some of the
> smushing in 'Ní·táet'.
In complex verbs, however, a prefixed preposition does not
automatically cause lenition. According to Thurneysen,
the prepositions ending in a vowel, the particles ro·,
no·, the interrogatives and the negative particles all
"geminate the initial of a following verb or verbal
compound, except in relative clauses." I'm not sure what
to make of that, except to say that my understanding is
that "gemination" as a third category of mutation along
with lenition and nasalization has not been universally
Note also that 't' is one of the letters in OI orthography
that regularly shows lenition, so that the in a leniting
relative clause we would expect to see "do·thét" (= that
Lughaidh has correctly pointed out that the OI pronunciation
of 'th' was /th/ not /h/.
> 3. Do we know what the difference in sound was between 'é' and 'áe'?
The latter is a diphthong, as far as I know, in words such
as "áes" (folk) and "gáe" (spear), thus /ais/, /gai/. But
in words that have 'ae' in a non-stressed syllable, such as
"cummae, ansae, daltae", the 'a' merely signals the broad
quality of the adjacent consonant, thus /kume/, /anse/, dalte/.
The stress in "ní·táet" is on "·táet", so I assume we have
a true diphthong here.