Hi Dave
If you heard someone talking nowadays and they said they came from Glenvar, they would simply mean the general area of land or valley of Glenvar and not one of the townlands within. These townlands would rarely be mentioned unless you would be living nearby and have local knowledge.
As for the Parish of Clondavaddog, where these townlands of Glenvar reside, you would very rarely hear it mention in a conversation also, unless you were getting married or a child Baptised in the Parish. Clondavaddog would have been used more readily as a title location in the early 1900s and 1800s and you will find it comes up very frequent especially if you were doing historical researching online, as all the old documents refer to Clondavaddog first and foremost, when looking at documents from back then.
The funny thing is though, although the name Glenvar is used all the time, whether it is nowadays, or going back years ago, you will not see it anywhere on any of the old research documents (1800s to early 1900s) as you will see in the links below, you may only see the townlands of the Parish of Clondavaddog (which is the land area of Fanad &Glenvar).
These would be the various Townlands found in Glenvar.
Tirlaydan Townland (Glenevar)
Carrowreagh Townland (Glenvar)
Lurganbrack Townland (Glenvar)
Tirevlin Townland (Glenvar)
Bunnaton Townland (Glenvar)
Clicking on any of these pages will show you a location map and helpful links at the bottom to view records back in the 1858 Griffith Valuations and 1901 and 1911 Census. You will see Gallagher families but McMonagles
Parish family names in the Parish of Conwal (Letterkenny) in 1850's
Parish family names in Clondavaddog back in 1858 (Note no McMonagles or similiar)
I suspect John McMonagle may perhaps have come from from the Parish of Conwal which is Letterkenny Town and Letterkenny west in the Barony of Kilmacrenen. P.s Barony is never referenced nowadays but remember they emigrated back in early 1800s, Baronies were very important location back then.
https://irelandxo.com/ireland-xo/news/what-parish
A Barony and Parishes would be explained better in this link above.
The unfortunate news however is that McMonagle is a very popular name and as you see yourself it is like finding a needle in a haystack, but chances are this is where they came from if recorded on his obituary etc..
I will reply properly on the rest of the query as soon as I can get a chance (up the walls busy). ;-)
Seamus