This is the sentence I'm trying to put into Irish -
- But as I have it tonght may ye have it
fifteen times better tomorrow night,
-Ach mar tá sé agam anocht go mbeadh sé
níos fearr cúig uair déag agaibh istoíche amárach,
(have I got anywhere near it?)

Looks more or less right to me. Couple of things:

Go mbeadh — I’d use either go raibh or bíodh (probably the latter)
Uair has a special plural form used with numerals: uaire
Oíche amárach is enough, I think; istoíche means ‘at night’.

Ach (an dóigh) mar atá sé agam anocht, bíodh sé agaibhse cúig uaire déag níos fearr oíche amárach

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Go raibh maith agat.
The next line begins with - or if not - nó .....mura.....?

Yes, I’d say so: nó mura mbeadh …, or nó murab amhlaidh ….

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

that's why I used mbeadh the first time, so the two phrases would have the same verb tense - is that preferable?

Not in this instance, so. They mean different things, so it doesn’t matter if they have different forms. If you think about it in French, you’d probably have different forms too (if you ‘forced’ a verb into the latter bit), right? Comme je l’ai ce soir, que vous l’ayez quinze fois mieux demain soir, but ou si vous n’aurez pas ça ….

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Well I can give the version in French -
Mais comme je l'ai ce soir que vous puissiez l'avoir
quinze fois mieux demain soir (I have to use the subjunctive - and I also thought that in Irish it's better to repeat the verb) but for the next sentence I should have given you the verb which is - caill, but in a negative subjunctive alors - ní gcaille sibh ?
or if not may ye not lose in the attempt - san iarracht

I've managed to find the original post
Braoin gave me this -
agus an scéal mar atá sé á inseacht anocht agamsa, go n-insí sibhse a sheacht n-oiread níos fearr an oíche amárach é...
insint = inseacht (which is what he hears storytellers say) and he doesn't believe in using fifteen times but rather seven (but fifteen times is what I have written down - it's a transcript)

"Twice" can also be faoi dhó.

Faoi chéad is "a hundred times".

How about faoi chúig déag for "fifteen times" ?

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Why not - if that works