Is "mbean" supposed to sound like "man" here? That's how uru works, right?
Have a look at this discussion, it explains it.
In Scottish Gaelic, "ag obair" can mean not only "working" but also "work" in a more continuous, long-term sense. E.g. if you wanted to say you worked in a bank, you could say "Tha mi ag obair ann am banca." Is that possible in Irish?
Is this more a sense of accusation, as in "I can't believe you're working with that woman", declarative, as in "oh, I see that you're working with the woman", or a command, as in "you're working with the woman or you're fired"?
I hope my question makes sense.
I think it's just a statement of what you are doing "right now" (working with the woman) vs. oibríonn tú leis an mbean (you work with the woman) which is something you do, but you might not be doing it right now.