The phrase , Srón fhada dhíreach , means a long straight nose.

Srón is feminine , so the qualifying noun gets a séimhiú.
Is the díreach also getting a séimhiú as it is also qualifying the noun ?

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Yes the following adjectives would all be lenited, and as far as I know the dental rule doesn't apply here.
Srón theann dheas - a strong beautiful nose

Although as always in Irish there are bound to be exceptions

dental rule?

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AKA "N blocking".

Sometimes an N at the end of a word can prevent lenition of dentals (sounds pronounced with the tongue touching the teeth). The dentals, the DeNTaLs, are DNTL. Because your tongue should already be touching your teeth to pronounce the N, leniting would be difficult, so often you don't bother.

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A language belongs to its native speakers, and when you speak it, you are a guest in their homes.
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The dental rule would apply in the speech of older Gaeltacht speakers, but it's gradually disappearing in this case.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Should have been dentals, sorry.

The DeNTaLS rule is about the dropping of lenition. It doesn’t really have anything to do with teeth, it just uses the word dentals as a mnemonic.
Any D;T; or S, that would in normal circumstances be lenited, is not lenited when the word before it ends in D;N;T;L; or S,
m.s. seanbhean but seanduine; rósghairdín but rósdearg, etc.. OK I’ve used prefixes there instead of separate words but the idea is the same.

Yeah, as MacBoo said: D,T,S don't get lenited after D,N,T,L,S:

cóta an dochtúra

Even though you would expect dhochtúra.

Although this rule is often ignored with adjectives:

Srón dhíreach


Do you mean younger speakers don't apply the dental rule as much, i.e. they would say cóta an dhochtúra; or do you mean they are applying it in the case of adjectives were older speakers didn't, i.e. Srón díreach?

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

i didn't mean in the case of the article, but on adjectives after nouns that end with a dental consonant:

older speakers: bean dubh
younger speakers: bean dhubh
etc

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Má tá suím agat i nGaelainn an aosa óig, do mholfainn "An Chonair Chaoch", tá tuairim daichead difríocht ann idir teanga na glúine is óige agus teanga na seanaghlúine, ach caithfead a rá gur botúin roinnt mhaith acu, mar shampla deireann a lán daoine óga "ag bualadh é" in inead "á bhualadh".

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

ah ok, so the rule doesn't apply because the dhíreach is separated by fhada.
if i was just to say straight nose, it would be srón díreach.

Something completely new to me

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