I suggested "They be working" since that is how I would say it (I'm from Tyrone). I remember reading an article, recently, about a group of black and white children in America being shown a picture of Elmo eating cookies while Cookie Monster looks on. Both the black and the white children agreed that Elmo was eating cookies while, when asked "who be eating cookies?" the white children replied Elmo while the black children said it was the Cookie Monster. Here, in NI, we'd say that Cookie Monster BES eating cookies rather than that he BE eating cookies but I thought this was a cool story and it's a nice demonstration of the difference between tá and bí.
The thing is, many of us don't speak that way and I'm not about to start. (Well, maybe I will because I kind of like the sound of it.) On the other hand, putting in an extra word that isn't there in the Irish ("regularly," for example), would also be problematic as well. If I see an English sentence that says, "I do be..." then I will know to put it into that particular form in Irish, but going the other way? Not likely.
The thing is, Irish has two expressions, "Tá siad ag obair" and "Bíonn siad ag obair", both of which mean slightly different things. In English, "They are working" would normally be used to express both meanings, and leaves it to context to tell which is actually intended. I find it problematic that the Duolingo lesson rejects a perfectly valid translation because it could be confused for another expression, and insists on a rather obscure dialectal phrase.
Our teachers (NY) using our text, Progress In Irish, teach that this would mean that they are continuously working and the do be means that in Irish.
Some of us remember our parents and grandparents from Ireland saying, "He does be going to work at 7am" - meaning he goes to work every day at 7am. But this is very confusing for Americans who have never heard such a concept in standard US English. Has to be really drilled in that we have no equivalent. And PII author always uses it with an I gcónaí or such to indicate the continuous sense.
There is no other way of transferring this to Irish. It refers to a continuing action in the present. Non native speakers would have a better understandnding than native English speakers as their grammar is more accommodating than English. Though modern English (Canadian, American, Australian & the RoI) grammar will accommodate. It is important to note that 'they are working...' is not the same