folamh go dtitead le ceart I láthair mo namhad = let me deservedly fall empty before my enemies

I'm just wondering what the role of "folamh = empty" is in this sentence.

Thanks!!

The sentence is being left shifted for emphasis: má dheineas olc ar an muíntir a dhein olc orm, [is] folamh go dtitead le ceart i láthair mo namhad = go dtitead folamh le ceart i láthair mo namhad, literally 'may I fall empty with justice/truth/honesty in my enemies' vicinity' = 'may I deservedly fall empty before my enemies'. Note: go dtitead is subjunctive (the 'wish case'), 'may I fall'.

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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

I’d say always without "is" (if the copula is meant)

… folamh go dtitead = empty I may fall

It is kind of a left shift but not a cleft sentence. (the latter: "gura folamh a thitfead")

I put the is in brackets because, though it is not said, it is grammatically what makes the sentence work.

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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

It works perfectly without the copula. It is not a copula sentence.