Pamela O'Neill wrote:
>  LL
>> Luid Lugaid Lâga úad ar cend ind f*âthlega 'sin Mumain. Tic-side cona thrí
>> daltaib leis. Co-cualatar éigem ind f*ir ic tiac[h]tain dóib dochum in tige.
>> "Cia hégim seo?" ar in liaig.
>> "Cnet do chlug," ar in dalta tôesech. [cf Keating IT viii 294: do cholg]
>> "Cráet so doridisi?" ar in liaig.
>> "Cnet do míl béo," ar in dalta aile.
>> "Crâet so dano?" ar in liaig risin tress cneit [error for dalta? cf next
>> word].
>> "Cnet do rind," ar in tres dalta.

> Lugaid Laga went from him to meet

or 'to fetch': cf DIL C 122.55

> the skilled doctor

literally 'wizard-doctor' (cf English witch-doctor). His ability as a 
'fáith' (seer) is an important aspect of his skill; one he is clearly 
passing on to his apprentices.

> in Munster.  The latter
> comes with his three pupils with him.  They heard the man's screaming coming
> to them towards the house.

Since Tadg is in the house, it is the doctor and his apprentices who are 
approaching the house, and the screams which are coming towards them. So 
'They heard the man's screaming while rhey were coming towards to house' 
(literally 'at coming by them towards the house'). For 'do' indicating 
the agent after verbal nouns, see DIL D 176.27f.

> "What screaming is this?" says the doctor.
> "A groan for a bristle of barley," says the first pupil.

'From' (= as a result of) rather than 'for'. The preposition 'do' here 
(and furtehr below) is for Old Irish 'di'.

> "What is this in addition?" says the doctor.
> "A groan for a living animal," says the other pupil
> "What is this besides?" says the doctor to the third pupil. [following
> Neil's correction]
> "A groan for a point," says the third pupil.

Looks good to me.

>> Lec
>> Luid Lugaid Laga uada ar ceand leaga sa Mumain. Luig Lugaid Lagha uada ar
>> cenn legha sa Mumain. & tic Fingin faithliaig cona tri daltaib lais sa
>> Sligig la Lugaid Laga co h-airm ambai Tadc mac Cein. 

 > I'm not sure about ambai - I've taken a wild guess but I'm not happy
 > with it:
 >  Lugaid Laga went from him to meet a doctor in Munster.  And Fingin
 > the skilled doctor comes with his three pupils with him in

Rather 'into', 'to'.

 > Sligo with
 > Lugaid Laga to the place in which Tadc mac Cein was.

Yes, indeed. Good wild-guessing. In 'a mbai', the initial 'a' stands for 
  'i', being the preposition 'i' with the relative particle (GOI pp 312 
and 521). This nasalises the following verbal form, which takes care of 
the 'm'-, and requires the dependent (conjunct / protonic) form of the 
verb (GOI p 28). The verb here is 'baí', the 3rd sg preterite of 'attá'.