"I bhfad ní ba fheargacha a bhí na thuismitheoirí" is this correct please?

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{Is geal leis an bhfiach dubh a ghearrcach féin}

Tuismitheóirí -= a modern word "progenitors" used for "parents", but hardly found in traditional Irish, which had a athair agus a mháthair, funnily enough.

Bhí a thuismitheóirí (na tuismitheóirí) níos feargaí féin (ní b'fheargaí féin).

Ní b'fheargaí féin a bhí na tuismitheóirí.

Ní ba - this is only really found in older literature, including traditional Cork Irish, and so it is good.

As direct translations are clumsy, you could consider:

Do bhí níos mó feirge féin ar na tuismitheóirí (tuismitheóiríbh)/ar a thuismitheóiríbh.

Is mó ná san féin an fhearg a bhí ar a athair agus a mháthair/ar an athair agus an mháthair.

(you don't have to repeat the preposition in tightly bound noun phrases)

[The last one is the best one.]