From An t-Oileánach

maran breagh atá'n lá laithithe agat orm, the English: 'you've made a fine muddle of the day on me'

In the only dictionaries that I could find that had maran, they had it listed as an alternative for mura. So is this mura breagh atá'n lá laithithe agat orm ('isn't it fine how you have damaged this day on me') or does maran have an alternative meaning (like 'muddle')?

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

Maran is the present tense of mara+copula. The past tense is marar.

If can be used in exclamations. From Amhlaoibh ÓL: "A Dhia mhuise," arsan gruagach, "maran deocair tu shásamh!"
This means "well, I'll be blowed/I'll be damned if you are not hard to satisfy".

Maran breá atá an lá lathaithe agat orm: (I'll be damned) if you have not mucked up my day for me!

Are forms without the final -n not found as commonly in Munster, though?

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

Well Peadar Ua Laoghaire doesn't use the forms in -n in writing. We don't actually know how he pronounced them, but AÓL is a better guide to natural Muskerry Irish.

Croidhe Cainnte Ciarraighe only uses maran, I believe

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar