The first two lines of this poem are:

Mo ghrá go daingean tú!
Lá dá bhaca thú

which means:

My love, securely you!
The day I saw you

On a tape I have of a recitation of this poem the speaker pronounces "tú" as "whoo" and "thú" as "too"

Based on the Text, I would have thought the pronunciations would be reversed. That is, "thú" = "whoo".

Am I right or is there some deeper principle involved?

Errare humanum est!
I think you're right. It's certainly how I would pronounce them.

I agree. It has been a great deal of time since I read or seen a copy of this lament; so either it is backwards in the print, or the recitation was wrong. I, however, am more inclined for the former, but not having seen the piece for years now I am not sure.

I checked the print in Duanaire - Poems of the dispossessed. It is as o'murchú gives it. And as a speaker of irish since the cradle, I'm fairly sure it's correct.

So you are obviously saying that the rendition that Ó Murchú has is incorrect, or were you just trying to be condescending, even though you had to check the print.

By checking the print version, Aongus was just making sure he had the facts straight. It sounds better the way Ó Murchú·has it, but tú or thú could have been used in either sentence. The recorded version that Ó Murchú has is wrong -- I'd bet my battered Ó Dónaill Dictionary on it -- but not terribly: Recordings are like the manuscript copies of old in that minor changes happen frequently.

Aongus was raised speaking Irish at home and he is also an engineer, and thus prone to meticulousness and maybe overchecking. I've taught Irish to engineers (those without Irish from the cradle, of course), so I know. At any rate, between Ó Murchú and Aongus I think we have good evidence that the Irish language is not wanting for cruinneas (precision) on the part of those who speak it!

The translation in An Duanaire of the two lines, by the way, is:

My steadfast love!/When I saw you one day/...

A great poem.

Míle maith agat Seosamh!
I am prone to double check things! And I hope I can avoid being condescending.
I've checked a second copy in Filidheacht na nGaedheal, so I'm now sure Ó Murchú's Text version is correct.

Beirigí bua

To all who checked, go raibh maith agaibh! I will now have to erase the incorrect pronunciation from my mind!

I think there's an easily stated logic to the pattern of using tú/thú in the Munster Irish of this poem: if the preceding word ends in a vowel, use "thú", otherwise use "tú". This mirrors the treatment of "dom/dhom, duit/dhuit" etc. in this dialect. Pointe eile, dála an scéil: bíonn an guta sna focail "tu/thu" gearr de ghnáth sa dán seo.