quote:Níl aithne agam ar daoine atá Gaeilge acu a thig liom labhairt leo.
Much of the sentence is correct but "who have Irish" is regarded as an indirect relative and so would be "a bhfuil Gaeilge acu"
Similarly "whom I could speak with" is also an indirect relative; a dtiocfadh liom Gaeilge a labhairt leo (referring to "daoine")
Aonghus and Lughaidh have already alluded to these.
To answer your question: Yes. The prepositional pronouns should agree in gender and number with their antecedents. Some grammatically masculine words commonly require the feminine but that is a different issue.
I would revise your sentence thus: Níl aithne agam ar dhaoine a bhfuil Gaeilge acu a dtiocfadh liom labhairt leo.
The preposition "ar" lenites the following d of daoine: ar dhaoine.