Which sense of “at work” do you mean — “at a work site”, “engaged in an activity”, or “laboring”?
Ag an obair could be used for “at the work site”, but it would be ambiguous. An unambiguous phrasing would be ag an láthair oibre.
I noticed this recurring problem where when "is" is contracted to 's, the answer isn't accepted. For example, I wrote "The keyboard's still working" and got marked wrong, presumably because it thought I took "méarchlár" for a genitive. Please fix this error! It's not just this sentence.
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oibríonn an méarchlár - "the keyboard works"
tá an méarchlár ag obair - "the keyboard is working".
Irish and English both distinguish between the simple present (oibríonn) and the present progressive (ag obair). They are not generally interchangeable.
I think that here, as it was discussed in the comments of other questions (including by you IIRC), it would again be better to say ag oibriú instead of ag obair since in this context it's working in the functional sense, wouldn’t it ?