I’m searching in Recordings using:
“Da Mihi Manum” AND work:"Tabhair dom do Lámh"
But now understand that Work is not an eligible field in Recordings.
Nor it appears, is Recording a eligible field in Works.
How do I hack together 2 searches:
One that will show me recording:“Da Mihi Manum” AND work:“Tabhair dom do Lámh”?
One that will show me recording:“Da Mihi Manum” AND work:-“Tabhair dom do Lámh”?
I don’t think either of these are possible to do right now. 
I would suggest making a ticket at https://tickets.metabrainz.org/projects/SEARCH asking for it. (Don’t hold your breath though. The current search server is unlikely to get any new features as we’re trying to move to a new search backend.)
I created Include Works in supported fields for Recording searches
I’m hoping that this venture into the world of tickets is better founded than my last. 
(I have taken anapneaic measures.)