About "tarbh" and "marbh", Ó Siadhail gives the pronunciation [ta:ru:] and [ma:ru:].
I wonder:
What is the pronunciation of these 2 words in the other dialects?
And for Dennis: what was their pronunciation in old Irish?

An Foclóir Póca puts a V sound on the end of both these words. I believe the FP is the standardized pronunciation. So your dead bull comes out as

taruv maruv.

Old Irish apparently didn't have the epenthetic vowel (an guta cúnta) that pops up in the modern language, so:

tarb /tarb/
marb /marb/

b = voiced bilabial fricative (Tá súil agam go ndúirt mé i gceart é!)

I believe that you'll hear /mar@b/ in Munster and parts of Conamara. The funny thing that happens in Munster is that the plural "marbha" is pronounced "marú".

The treatment in Scottish Gaelic is interesting: /m[ara]b/, where the unit in square brackets is conceptually a single syllable with equal stress on both vowels, at least for poetic purposes. Here the guta cúnta normally echoes the vowel that comes before it.

Donegal: /taru/ and /maru/.

The final -v sound is only to be heard in Munster, I think. It is a short oo-sound elsewhere.

"Final non-palatal bh after an epenthetic vowel is vocalised to u: or less commonly retained as w, e.g. marbh ma:ru:, ma:rэw, banbh ba:Nu:, ba:Nэw."

— Tomás de Bhaldraithe, "The Irish of Cois Fhairrge, Co. Galway"
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"Epenthetic vowels occurred at the intermediate stage of the development of tarbh and such words — e.g. balbh balu:, garbh garu:, banbh banu:, searbh ∫aru: — thus tarbh → *tarabh → tarú. So too an epenthetic vowel developed between r and gh at an intermediate stage of the evolution of dorgha, thus dorgha → doragha → *dorú → drú."

— Éamonn Mhac an Fhailigh, "The Irish of Erris, Co. Mayo"

(Message edited by Peadar Ó Gríofa on October 15, 2005)

"It is a short oo-sound elsewhere."

Short in Achill as it is in Donegal, but long elsewhere in Connaught. That is, "half-long," like any other unstressed "long" vowel.