Is 'nú' an older form of 'nó' ? Here's the context -

AN CHROCH GHEAL

Ní mise cheap. Ní fhuil agam-sa acht mar a chuala. Ní chuala acht mar adubhradh, agus ní dubhradh acht bréaga nú éitheach ar fad.

It's not in the Dinneen dictionary.

Looks like it's a pronunciation spelling of "nó"

Aye, it's the Connemara pronunciation of "nó".

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

I think it must be Munster Irish too - go raibh maith agaibh. And what about the first sentence - Ní mise cheap. What does that mean? - I myself I don't think ? I'm not sure about that either, is - cheap - used as a verb or perhaps even a noun? I'm guessing that - adubhradh - and - dubhradh are Munster forms of the verb - to say.

I read the first sentence as "Ní mise [a] cheap". Basically, "It wasn't me who thought (it up)"

I'd say:
Ní mise a cheap = It's not me who thought (so)

Go raibh maith agaibh - yes, that makes more sense of it. It's one of the stories in the storybook 'Sgeuluidheacht Chúige Mumhan' compiled by Pádruig Ó Laoghaire, which you can download from Internet Archive. Incidentally it's supposed to be 'Part One' so apparently he didn't get round to publishing any more.
I see that his preface to it is quoted (in English) by Philip O'Leary in his book 'The Prose Literature of the Gaelic Revival 1881 - 1921: Ideology and Innovation' and that Douglas Hyde fully agreed with what he had to say about the oral tradition.
If anyone is interested, here it is -

Roimh-Rádh

Ag clos mórán sgeul dom agus mé go han-óg adubhart féin dá mbaileóchaidhe iad, agus iad do chlóbhualadh go dtairbheochadh sé d'á lándhóidh-seo go háinthe ná cuala iad. B'fhiosach mé go rabhadar luachmhar, óir taisbeánaid dúinn intinn agus intleacht na nGaodhal níos soléire solusmhaire, b'fhéidir, ná aonnidh eile atá fástha anois againn, agus aonrud shoillsigheas ar a nósaibh-san is cóir dúinn bheith suimeamhuil ann. Má's blúireacha iad féin agus blúireacha truallighthe, ar uairibh, is oighreacht shaidhbhir iad i n-a dhiaidh san.
Cionnus nár cuireadh cúramh ionnta i bhfadh roimhe seo ní heolach domhsa. Luthgháirim go ndeárnaidh an "Chraoibhín Aoibhinn" níos mó ná aonneach eile mo bhrostughadh chun cnuasach na sgeul so. Ní gur iarr sé orm a dhéanamh, acht 'nuair léighios ar chruinnigh sé féin díobh, tháinig fonn orm feuchaint le beagán d'á luighead, 'nuair nach raibh aonneach eile ag claoidhe chuige.
Bhí a fhios agam go raibh mianach comh saidhbhir agamsa im' chúige féin agus do bhí aige-sion i n-a chúige féin. Fágaim faoi'n léightheoir a ndeárna mo ghnó maith nó olc.
Is le lucht léighte na sgéal-so luighios cló-bhualadh tuille d'á leithéididhibh, óir díoltar a dhian-luighead-san de leabhraibh Gaedhilge gur misde do'n té chaitheas mórán d'á aimsir leó.
Ní fuaras acht síbhialtacht, carthanacht, agus féile ó'n mhuintir thug na sgéalta dham. Do Dhomhnall Ó Néill, thar agus os cionn cách eile bheirim buidheachas ó chroidhe.
Do gach n-aon do chabhruigh liom i leith sgéil, airgid, agus uile táim an- bhuidheach. Ámh muna mbeidheadh an lucht do dhíol as tabhairt-imach an leabhair-seo ní fheudfainn-se a fhaiséis go fóil, nó b'fhéidir eile go deo.
Do léigh Seaghán Ó hOgáin na fromhtha dom agus bhí comh maith agus na focail agus na ráidhte ba dhoigh beis bheith do-thuigthe do'n léightheoir coitchionn do chur i n-iúl dom.

Pádruig o Laoghaire.

My old school teacher/principal from primary had excellent Irish (I think he was from Clare? whether he learned it there is another story but I believe he was a Munster speaker - I could be wrong about that though) and pronounced "nó" as "nú" for what it's worth, so you are probably right about it possibly being a Munster thing too.

I should have checked the glossary and notes at the end of the book - it's explained there at the bottom of page 121. Here's what it says -

Nó : Nú

I n-áit "nó" deirthior "nú" i Mumhain. Fós níor casadh aon eisiompláir de liom 'san litridheacht. B'fhéidir ó sontar "nua," .i. úr, mar "nó", go ndeánadh an t-athrughadh-so, chor go bhfeudaidhe an dá fhocal do dheifirughadh go soilléir.

This storybook contains the following stories -

Na Trí Preacháin
Páidín Ó Dálaigh
Rí na mBréag
Oileán na gCúig mBeann
An Chroch Gheal
An Fiolar Fealltach
Máire Ní Rógáin

plus Gluaiseanna agus Notaidhe

The seanchló print is clearer than in the individual books that I've found on Internet Archive or that I've managed to buy and it's also available in Roman print on the RIA website. At first I didn't realise that this book contained them all - I was under the impression that it was just another book in the same series and I even managed to mix him up with the other Ó Laoghaire.
I've even had David Webb (he of CorkIrish wordpress) writing to me about it. All I need now is a 19th century Munster Irish dictionary and list of verbs to help me understand it more easily.

"I didn't think (so). I only have what I heard. I only heard what was said and what was said was nothing but lies and complete perjury." I don't know if its Munster dialect, but this is what i got out of it. Wait for a better effort, though - I'm a relative beginner.