Can't remember whether this was on A or B so I'm posting to both, apologies.
Coming back from a wedding in Blair Atholl we went for a walk in Pitlochry and came across a sign talking about the place Fonab, giving this as Fonn Aba 'Abbott's Land'. Which made me go on another trawl but this time of Latin dictionaries and I think I found another clue as to the (original) meaning of fonn in Gaelic.
The meaning of fundus in Latin seems to have been not so much ground but an "estate" - that is, a piece of agricultural land containing a building (for example Perseus gives "fundi appellatione omne aedificium et omnis ager continetur").
Which I think makes sense as the church would own lots of good land so if the term got borrowed, it's logical in a way that it would end up as a fancy word for "land" as different from the more common "ùir" and "talamh" (neither of which imply buildings). Given enought time, the original meaning may well have gotten watered down.
At least another example for Colin's database ; )
Am Mìcheal Eile