What is the difference between "mura" and "muna"? I have a sentence :
Mura bhfuil An nó Na roimh ...
I understand it means " if there isn't .."

I haven't a sentence for "Muna". Maybe someone could give me an example of both uses and explain if they are interchangeable or why one is used over the other. Thank you

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{Is geal leis an bhfiach dubh a ghearrcach féin}

Depends on the dialect, some dialect use "mura", other "muna", others "mara" etc.

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

If you want examples of usage, the pota focal website is good for that. An example that site shows for muna: muna mbeidh ar a gcumas, if they're unable. But mura and mara would be fine here too (and both used in Cork Irish).

muna = mura = mara (even = mana) = if not
Standard Irish is mura.
Etymology: mani (má ní) > moni, muni, muna > mura, mara

Often two or more forms are used in the dialects.
It is hard to assign them to a particular dialect.

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{Is geal leis an bhfiach dubh a ghearrcach féin}

For the Déise, I have only been able to find examples of mara(n) being used. I don't believe that the other forms are found in the dialect.

Does anyone know what is used in Ulster and Kerry?

I also found on Wiktionary that mara is supposedly used in Cois Fharraige, but I'm almost certain that I saw somewhere that muna is used in Cois Fharraige.