Certainly mistakes.
Cad is not a relative pronoun.

Bhí aimhreas orm faoin rud a bhí á dhéanamh acu.
Sin (mar) a cheap mé.

Sounded weird to me too. However, I think I saw a few sentences in Tobar na Gaedhilge (traditional literature, especially from Donegal) with such use of interrogative words, so I guess it's possible. But it's probably an Anglicism anyway.

One can find for instance "Chuala mé trácht ar caidé mar tháiniceamar go Gleann Cártha an chéad lá ariamh".

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Agus is í Gaeilg Ġaoṫ Doḃair is binne

Following verbs or verbal nouns like trácht (mention) caidé mar is probably justified as an indirect question.

Thanks, Labhrás and Lughaidh for your helpful input!

I don't think it's an indirect question.
An indirect question is sthg like "I don't know what he did" etc.

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

I don't see any difference between "I don't know what he did" and "I heard mention of how we came to GC..."
Both of them have indirect questions: "What did he do?" "How did we come to GC?"

You could easily switch the first part of either with the other.
"I don't know how we came to GC" or "I heard mention of what he did"
"Níl sé ar eolas agam caidé mar tháiniceamar go GC..."
"Chuala mé trácht ar céard é a rinne sé"

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