Hello Kyle,
I think you cannot rule anything out at this stage. The absence of a McGreal and O’Donnell marriage with the right Christian names and the records that do exist upto 1911 may simply be indicative of human behaviour of concealing a family secret.
Pursuing the DNA line will hopefully allow you to focus on which group of McGreal ancestors to connect with. Fortunately the Westport McGreal area with its large diaspora in the US has many of this family researching their ancestry.
As for the Castlebarnagh, Kilconduff using the Griffith valuation in itself is not strong enough to base a connection. It is a census substitute and does not indicate the age of the head of household and the parish records show little to be encouraging. However if you have unlimited funds you can apply to the http://www.valoff.ie/en/Archives_Genealogy_Public_Office/ for details of John McGreal and his holding. This should indicate when changes in tenure on the land occurred suggesting a possible period of death and who took over the tenure as the family do not appear in the 1901 census. The records are known as the revision books or the cancellation books.
The Oughaval McGreal people as mentioned are the largest and well researched and the town Westport often drew other McGreal families from other areas throughout the late 19th century.
Several of the Westport families were engaged in the butchery trade and often members would attend fares as far afield as Ballaghaderreen which is now in Roscommon. In one instance I came across a family of ten orphaned from Westport area who were sent to extended families outside the town , some of which were sent to an aunt who had married into a family in Ballyhaunis in East Mayo.
As for the location Altamount Street and Prospect. The street is fairly easily identified but I feel the Prospect is not the location you have identified. In the parish records you will find the names of placenames which do not appear as townlands as we recognise them when used for civil records. There is an area known as Prospect which straddles the townland Fahburren, and Cloonagh Co. Mayo south of the town Westport. - https://www.google.co.uk/maps/place/Fahburren,+Co.+Mayo,+Ireland/@53.7668311,-9.619302,14z/data=!3m1!4b1!4m2!3m1!1s0x48597c60bc852021:0xcf7c77c0f8425614
Rootsireland may offer you a means to spend money but I would advocate that you trawl through actual parish registers to avoid transcription errors that may occur with rootsireland. Focus on the McGreal family of course but also any clues that may be shown with the address field focussing on Altamont St and Prospect. http://registers.nli.ie/parishes/1043
Once you have a DNA sample that allows comparison I would reach out to the Facebook Group https://www.facebook.com/groups/clansofmayo/ who have several McGreal researchers you can go behind the wall of online records and discover the more subtle family histories which may offer you an answer.