I’d guess if you really want to express I am at work (as different from I am working) then perhaps tha mi aig obair since ag in Gaelic is really only used with the progressive verbal noun expressions? But I’m not sure. In Irish both would be the same.
To express that you are in your work-place or that it’s your working hours, you could say exactly that: I am in the office, it’s my working hours, etc. – so rephrase it somehow.
tha mi ag obair by default would be understood, I think, only as I am working.