'They are speaking Irish' would be 'Tá siad ag labhairt na Gaeilge'.
True, but it's the English translation that's unnatural - not the Irish. It would really odd to use the passive like that in English.
I is normal for native Irish speakers to normally speak in the passive form (such as this sentence)?
I don't understand how this turns into "being spoken" when on the other sentences we have had it would have been "to speak"