"Well, that's a hard one," says the King of Ireland's Son.
"Indeed it is," he said, (Féidlim Tonn Rí) "I don't think you'll do this one."

"Bhuel, is deacair an ceann seo" arsa Mac Rí na hÉireann.
" Sea, go cinnte," arsa duine eile, "ní cheapaim go ndéanfaidh tú an rud seo."

but could I say instead - "ní cheapaim go n-éirí tú leis an rud seo a dhéanamh." ?

if you want to say "I don't think you'll succeed in doing this" it would be:

Ní cheapaim go n-éireoidh leat an rud seo a dhéanamh.

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

Go raibh maith agat.

Now, I hope you've enjoyed it and that you'll depart in radiant contentment.

Bhuel anois, tá súil agam go mbaine sibh sult as agus go n-imí sibh go sona sásta is go gealgháireach.

No subjunctive mood here.
… gur bhain sibh (gur bhaineabhair) … agus go n-imeoidh sibh …

Go raibh maith agat.

I'm not sure about this one either -

And all I got out of it
was stockings of stir-about and brogues of buttermilk.

Perhaps - Agus ní bhfuaireas dada ann ach
stocaí leitean is bróga bláthanna.

And that's what got me this far. - Agus is iadsan a thug mé go dtí seo.

This is why I prefer to only speak Irish with Irish people -- their English is really something else.

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ЯГОН ТОҶИК НЕСТ ИНҶО???

That's a strange one.
I wouldn't use the genitive as the socks aren't made of porridge etc.
Maybe: Ní bhfuair mé as ach stocaí le leite (iontu) agus bróga le bláthach (iontu). But I think you can leave the pronoun out and still the meaning is clear.

And that's what got me this far = Agus sin a thug chomh fada seo mé.

Go raibh maith agat.
I was always under the impression that these were meant literally, ie - that the stockings were indeed made of stir-about and the brogues made of buttermilk. When I originally asked about this, a long time ago, at the other place, Scoobytyson gave me - stocaí leatan - for the first one. Perhaps your understanding of it is that they were filled with stir-about and buttermilk. In either case, it wouldn't have made walking in them very easy, but that's storytelling for you.

Go with whichever meaning you think yourself. If it's a fairytale then I suppose anything is possible.