"Tá sé ag déanamh tae."
Translation:He is making tea.
@Cathal, to answer your question, it would be the same difference as between making and preparing.
Ullmhú is to prepare, so it could be acceptable, but it may not cover the same meaning. You can prepare food without making it. But sometimes it is the same. You can prepare tea, but once my husband told me it was not made before it was brewed+milked+in front of him: I had only prepared it. Another day he might argue the opposite!
You prepare a statment, before making it...
Could be — I hadn’t seen ag ullmhadh before. Does it have a Munster origin?
I don't know its origin but it turns up here too: http://digital.nls.uk/early-gaelic-book-collections/pageturner.cfm?id=82316806&mode=transcription I've always assumed '-adh' and '-ú' are they same, just with the second one shortened for easier spelling.
That link points to a book that was published with a pre-reform spelling, so ullmhadh is likely the pre-reform spelling of ullmhú. I hadn’t seen the spelling of ag as aig before, even in Dinneen’s dictionary, so I thought at first that that might have been a Scots Gaelic text.