Would there be any difference in using this over the likes of "tá siad ag téamh an tae"?
I think so. The way I've always heard it is that the sentence here is more 'The tea is being heated by them', so it's a passive, where as the other is 'They are heating the tea.'
I take it, then, that the passive voice isn't as verboten in Irish as it is in English?
What's the difference between 'the tea is heated by them' and 'the tea is being heated by them '. If this example is only 'being heated' how is the active form translated?
In English, the difference between "the tea is heated by them" and "the tea is being heated by them" is that the first construction has a habitual aspect, whereas the second describes what is happening now. You can restate them as "they heat the tea" and "they are heating the tea".
I think that tá an tae téite acu, using the past participle of téamh, is the construction that you are looking for, though the saorbhriathar would probably be preferred in many instances.