The needles indicate when there's a current being supplied to the instrument from the cabin at the far end of the corresponding section, allowing the signalman to remove the staff. When there's no current, the needles point 12-6, and when there is a current they move off-centre (1-7 or 11-5).
(And because they're powered by the same dynamo that powers the bell signal system, the needles twitch whenever the cabins are commmunicating)
Decent photo of the staff instruments in Claremorris cabin here: http://www.signalbox.org/overseas/ireland/claremorris-i.jpg