In conclusion on this item It would appear that the James of High St deceased in 1889 could be the son of Austin McGreal and Judith Scott baptised 1850 Oughaval Parish. The age at death would be consistent with the baptismal date.
The absence of any other death record for a James prior to the date of marriage would suggest a likely date of death being prior to 1875. The likelihood of any death record prior to the 1874 Registration Act is unlikely , the only record would be a headstone if it still existed.
It is unlikely John is connected to the town of Westport but would have associated with it like many other parts of Ireland being the major center of population to his actual location .
The McGreal/O'Brien family remains the only remaining family unit we can possibly make a link with given the possible age of John and his fathers name not only in the general area of Westport but also the remainder of Mayo.
It cannot be confirmed if any of the other siblings survived to the 1901 census or if they emigrated. The two other brothers Austin and Patrick draw blanks in the US and Uk. There is a possible marriage to a John McKeon 10 Nov 1889 her parents being a James and Bridget approx dob 1859 in the US. It may be impossible to attribute this Bridget to the family under consideration .
A family unit which appears in the 1901 census in
Boleybrian,Knappagh a townland close to Westport is comparable in ages to the McGreal/O'Brien family without Bridget the daughter and the inclusion of Austin's wife.
From the Griffith Valuation extracts 1855, Boleybrian was a townland exclusively occupied by McGreal families. Two of these were a James and a James jnr possibly suggesting father and son having land holdings.
From the IFHF site possible marriages of a Austin,Bridget or Patrick with a father James upto 1900 turned up one possible in Annagh/Ballyhaunis for a Patrick who was born 1866 to a Francis/James McGreal and Catherine Kilduff. Austin clearly does marry from the 1901 census entry and this was to a Winifred Maly in 1895. The fathers entry appears to be a Tom from the transcribed record, however records on IFHF Mayo show the only Austin born in Mayo to a Thomas was in 1876 to a Thomas and Bridget O'Donnell. It may be the case that Austin grew up knowing his father as Tom to differentiate between Grandfather and Father which was common with identical names in families.
Austin and Patrick were born about four years apart. The 1901 census indicates a three year gap in ages. Of all the Austin McGreal born between 1850 and 1859,regardless of fathers christian name only two families contain a Patrick, The other is born to a Patrick and Catherine Conway and is seven years older than Austin, rather than four years younger with the McGreal/O'Brien family.
The IFHF Mayo site is a considerable help in research. If you choose to look into this family in Boleybrian I would suggest however you source copies of the actual records from GRO Roscommon as errors in transcribing do occur.