Shandog,
John Shannon’s Scottish 1887 birth certificate should record his parents names and the date and place they married.
Here’s the 1879 marriage certificate in Woods Chapel (Church of Ireland). William was a labourer at the time and living in Ballymahon (Ballymoghan).
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1879/11055/8044435.pdf
His father is named as John Shannon, also a labourer.
Here’s a death for a John Shannon in Ballymoghan in 1872, aged 76. He was a farmer and unmarried but I suspect is related to the family. Informant was John Shannon of same townland.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1872/020751/7280137.pdf
Another William Shannon died in the registration area in 1866 aged 94, but that is not on-line free yet. You would need to pay to view on the GRONI site. That may well be William father of Samuel.
John Shannon’s death in Magherafelt workhouse in 1885. He was 84 and still married. He lived in Ballymoghan and was a labourer but there’s nothing to confirm who his parents were etc.
I couldn’t find a death for his wife.
Death of Mary Shannon in Magherafelt workhouse in 1889. She was aged 83, from Ballymoghan and a spinster. Again probably related to the family.
I don’t see a death Samuel Shannon so suspect he died before 1864 when death registration started.
There were no Shannons left in Ballymoghan by the time of the 1901 census.
The 1831 census lists just 1 Shannon household in Ballymoghan Beg. That was headed by William and consisted of 4 males and 3 females, all Presbyterian. They may have attended Magherafelt Presbyterian church which has records from 1771 onwards. Or Castledawson which has records for 1703 to 1706 then 1809 onwards. Copies of both sets of records are in PRONI on microfilm. So personal visit required to view them. Marriage below suggest they attended Magherafelt.
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Loughinsho…
1862 marriage for Ellen Shannon, likely brother to William who married Charlotte. A William was a witness:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1862/09634/5494314.pdf
Tradition was to marry in the bride’s church which is why I think it was probably the Shannon family church.
Samuel Shannon marriage in 1863 so a sibling to William and Ellen.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1863/09654/5501393.pdf
Thomas Shannon marriage in 1861. He was a widower so may be a bit older than expected. His father was Samuel. Register Office ceremony so they might have been of different denominations.
Thomas Shannon’s 1st marriage in 1851:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1851/09413/5410309.pdf
John Shannon marriage in 1875. Again a widower, son of Samuel:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1875/11192/8101424.pdf
That’s all I can see from a quick search No wills on the PRONI wills website. No mention in the PRONI name index. Don’t see any mention of the family in the newspapers.
I’d search Magherafelt church records next to see if you can find baptisms and pre 1845 marriages for the family.