It would refer to the eye condition, long sightedness. How about this from the DVLA website: "unrhyw gyflwr sy’n effeithio ar y ddau lygad (ac eithrio golwg byr, golwg hir neu ddallineb lliw" which means "any conditions that affect both eyes (except short sight[edness], long sight or colour blindness".
Having said that, I have seen the phrase used to mean "a long look" too, as in "take a good long look at this document". Not sure if that is accepted as an alternative or not.
Sorry, My first comment misunderstood what you are saying.
In the UK, long sightedness and far sightedness are not the same thing. Long sightedness is Hyperopia - the condition that affects everyone when they get older and need reading glasses. That is always and pretty much only called long sightedness.
Far sightedness is something different. That is the ability to see into the far distance. It would be a mistake to refer to that ability as “long sight”. I am not sure golwg hir is right for that. I will ask a first language speaker and come back on that one.
I've never heard the term long sighted for presbyopia, so thank you and Duo for the english vocabulary expansion! Generally such a condition would simply generate comments on the need for reading glasses. Hyperopia, however, is the medical phrase for far-sightedness. I LOVE the term "away with the faeries!" so for that I also thank you!
OK just asked a first language speaker who does translation work... and he could not think of an exact synonym for far sightedness. He would say “gallu gweld yn bell” (able to see far).
He also cautioned against using golwg pell, which is a figurative construction that can mean “away with the fairies”!