No, I don't think so.

There are different kinds of constructions with multiple genitives depending merely on definiteness.

In case it were "deireadh na seachtaine" everything would be clear: No genitive form of deireadh or ceiliúradh but lenition.
Easy rules because all units of meaning are definite.

But here deireadh seachtaine is indefinite. Neither "deireadh na seachtaine" nor "an deireadh seachtaine".
So different rules apply:

I'd simplified the construction
seoladh ár gceiliúrtha deireadh seachtaine = the launch of our weekend celebration

[Deireadh seachtaine] is in genitive relation to ceiliúradh. But deireadh doesn't change into its genitive form deiridh
because both deireadh and seachtaine are indefinite and seachtain is already in genitive form.

So the remaining question is:
What happens to ár gceiliúradh which is again in a genitive relation to seoladh?

Here again, something is different: Ár gceiliúradh is definite, but its attribute is indefinite.
That means, ár gceiliúradh must be in genitive form:
seoladh ár gceiliúrtha deireadh seachtaine = the launch of our weekend celebration

EDIT: -tha instead of -aidh

BTW: Maybe deireadh should be lenited following a masculine noun in genitive but I don't know. It sounds strange.

I have always found this to be very confusing. Not the matter of definite versus indefinite, but rather the specifics of "deireadh seachtaine"

so are these all correct:
Céard a rinne tú le linn deireadh na seachtaine? what did you do during the weekend
Céard a bheas ar siúl agaibh ag deireadh na seachtaine seo? what will you be up to this weekend
Céard is maith leat a dhéanamh ag an deireadh seachtaine? what do you like doing on weekends
Nach dtiocfaidh tú deireadh seachtaine éigin? won't you come some weekend
Is maith liom an deireadh seachtaine. i like weekends

and if not what would be the correct version? I find this very difficult for some reason.


the genitive of ceiliúradh is ceiliúrtha (it is a verbal noun)
I believe that this lenition only happens after slender masculine genitives, so since "ceiliúrtha" is not slender it would not be lenited.
And when we add bliantúil, even though it now is a slender masculine genitive ending, it does not séimiú because of L and D being incompatible

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deireadh seachtaine = a weekend, an end of a week
deireadh na seachtaine = the end of the week
an deireadh seachtaine = the weekend
But there's no big difference between them.



Oops
Yes, of course: ceiliúrtha.



bliantúil as an adjective doesn't have any force to cause lenition.

Hmmm... is this wrong:

tosach an cheiliúrtha bhliantúil ghrá
the start of the annual love celebration


also it looks like I was wrong before, I think that ceiliúrtha would still lenite?

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Yes, it lenites.
All masculine nouns in genitive singular cause lenition in Standard Irish.
But in nominative plural only nouns ending in a slender consonant cause lenition, e.g.

nom. sing.: an fear mór, an múinteoir mór, an bean mhór
gen. sing.: an fhir mhóir, an mhúinteora mhóir, na mná móire
nom. plur.: na fir mhóra, na múinteoirí móra, na mná móra

And to complete this:
gen. plur.: na bhfear mór, na múinteoirí móra, na mban mór
voc. sing.: a fhir mhóir! a mhúinteoir mhóir!, a bhean mhór!
voc. plur.: a fheara móra! a mhúinteoirí móra!, a mhná móra!



BTW: Of course, in dialects genitive forms are often not used at all.

Thank you for the clarification about séimhiú, but I was wondering if your yes was referring to that (séimhiú) or to whether that example was wrong

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... an cheiliúrtha bhliantúil ghrá is correct because ceiliúrtha causes lenition.

So if the genitive noun itself did not cause lenition but the agreeing adjective were still slender-ending and another adjective or genitive followed, it would not be lenited?

It was my impression that lenition always relates to the previous word and that separating a word and its reason for being lenited (with an adverb, for example) would block lenition

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No, lenition according to this impression would only be a phonological phenomenon, a kind of sandhi.
It was so, historically, and still is so in some circumstances - but usually today it is a grammatical device e.g. a part of declension, no matter of the phonological circumstances:
genitive masculine causes lenition, an fhir [....] mhóir dheas
nominative feminine causes lenition, an bhean [...] mhór dheas
Often even so if lenition would be avoided phonologically, e.g. it is "an bhean dheas" in Standard Irish (and Connacht at least), though the sequence -n d- should normally hinder lenition.

Adverbs often block declension and so of course they block lenition as well.
it is "srón mhór" (a big nose) but "srón cuibheasach mór" (a quite big nose) because mór isn't declined for case and gender following the adverb cuibheasach.

Hmmm... these seem a bit contradictory. I realize it isn't just sandhi anymore, but why do adverbs block then?

Is that really standard? Yuck. Don't tell me an bhean dhúthrachtach is correct, is it?
I thought the dntls rule was a sandhi rule holy above all other grammar.

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