This is from Cnósach Focal ó Bhaile Bhúirne:

Maybe this means "the mare was trying to kill the foal, and we devoted the night to them, to see if she would accept the foal".
What does the suas leó mean here?

We spent the night with them?

I thought of "spent the night with them", but why include suas? It seems some of these phrases can have an optional directional complement (suas, síos etc) thrown in randomly?

I would have read it like Tim, "we spent the night".

In Hiberno-English, you get "give up" with a meaning something like "endure". The notional origin being to "offer up" one's suffering as a form of penance, often specifying the intended beneficiary. I've heard people say of inconveniences like a delayed plane "we'll give it up for world hunger" or "we'll give it up for those affected by the flood/earthquake, etc." Perhaps this usage in English is related to the Irish "tabhair suas", and if so it could be read as "we offered up the night spent with them, seeing would she accept the foal".

The closest approximation I can think of in standard English would be "we spent", only it doesn't necessarily have the connotation that the activity in question was perceived as unpleasant or irksome. That connotation has to be inferred from context instead, however, in this case the context would certainly suggest that. So I'd say "we spent the night with them to see if..." would be a reasonable translation.

Thank you, Ade and Tim.

Keep posting these interesting little questions. They stretch the mind a bit.