The “works for” thread is getting a bit long.
Daithí is correct that in this article in Beo Pól Ó Muirí uses “ag cogaint air” in this sense.
I have great respect for Pól’s work as a journalist: but he himself would be the first to admit that he is not a native speaker. Beo is an extremely well edited source, but most of the writers are not native speakers. So a single instance in Beo is not evidence. Nor are several instances, if they come from the same author.
By the way, for those who are interested in such matters, the Corpus of texts assembled as part of the new English-Irish dictionary project is available for use at http://corpas.focloir.ie/ (Free registration required, doesn’t like the Chrome browser, uses popup windows for login)
So, my suggestion:
Cogitating over something is “marana a dhéanamh ar rud”