This is one I did earlier - quite a long time earlier in fact, but I'm not at all sure how to use this construction with - mura. I'd be grateful for some help with it please.

But as I have it tonight may ye have it - Agus an scéal mar atá sé á inseacht anocht agamsa,
fifteen times better tomorrow night, - go n-insí sibhse a chuig huaire déag níos fearr an oíche amáireach é
or not may ye not lose in the attempt - mura......san iarraidh/iarracht
but the front teeth or the back teeth, - ach na chlárfhiacla nó na chúlfhiacla
a slice of the jaw - slisne an ghéill
or a lump of the gum - nó cnapán an charbaill
or maybe the tooth that's furthest back in your head - nó b'fhéidir an fhiacail is sia i gcúl i gceann agaibh
to be for a staff in your hand. - a n-éirí bachall i láimh.

Mura (muna, mara) is the negation of má and dá - If not…

Mura ndéanfaidh tú an obair, ní bhfaighfidh tú an t-airgead.
If you don’t do the work, you won’t get the money.
Mura ndéanfadh tú an obair, ní bhfaihfeadh tú an t-airgead.
If you didn’t do the work, you wouldn’t get the money.

I think in Ulster they use the present and the past subjunctive instead of the future and the conditional, but I'm sure one of the Ulster people will straighten that out.
Mura ndéanann tú an obair…..
Mura ndéanadh tú an obair…..

If you’re translating may you not…then that is nár + subjunctive

Nár dhéana tú an obair agus nár fhaighe tú an t-airgead.
May you not do the work and may you not get the money.

I think I misunderstood your question.

Do you want to say ..or if not may you not lose..?

..agus mura mbeadh sin amhlaidh, nár chaille tú....

might be one way of saying it.

Go raibh maith agat. In fact looking at it again, I probably shouldn't be using mura but rather the subjunctive with the negative - nár -
nár chaille sibh san iarracht ach......may ye not lose in the attempt but....
Is that correct? I'm putting it in the plural because he's addressing his audience and he's using ye rather than you.

Here's another question I have - when you add scoth in front of a word, it is the equivalent of the ending -ish in English - such as longish - scothfhada, though bluish is cineál gorm. I've found scothaosta for - oldish, but can I say scothbheag for - smallish, it isn't in the dictionary as such, so I'm not sure it exists. This is the phrase I want to translate -

And he was a smallish, oldish man

Can I say? :

Agus bhí sé fear scothbheag, scothaosta

or

Agus bhí sé firín beag é agus scothaosta