The tithe applotment records for 1829 show just one Shannon farm in Ballymoghan. That was held by a Samuel Shannon.
https://www.irishgenealogyhub.com/derry/tithe-applotments/magherafelt-parish.php
By 1831 he’s gone - dead perhaps - and replaced by William referred to in my earlier message. (Household contained 4 males & 3 females, all Presbyterian). Perhaps Samuel’s eldest son? Then by Griffiths Valuation (1859) there’s still only 1 farm (plot 9) but it’s split between Robert, William, John & Thomas. So presumably that’s the John who was William’s father. They may well all have been brothers.
That John died in Magherafelt workhouse in 1885 aged 84. His wife was still alive (but she was his 2nd wife. See marriage details below):
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1885/06302/4806501.pdf
In the Valuation Revision books on the PRONI website, all 4 Shannon tenants on plot 9 in Ballymoghan Beg were deleted in 1883 and replaced by Andrew Black. So they had given up the farm and at least some were probably dead.
Here’s Williams death in 1872 aged 76. Unmarried. Informant was brother John.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1872/020751/7280137.pdf
Robert may have died in 1866 aged 77 and Thomas in 1865 aged 72. (Those last 2 are not on-line free yet and are pay to view on the GRONI website).
Here’s an Ellen Shannon marrying in 1862. Looks to be a sister to your William.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1862/09634/5494314.pdf
Samuel Shannon marrying in 1863. Looks likely to be a brother to William and Ellen.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1863/09654/5501393.pdf
Thomas Shannon, widower, remarrying in 1861. His father Samuel.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1861/09614/5486937.pdf
Here’s his first marriage in 1851:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1851/09413/5410309.pdf
John Shannon, widower, remarrying in 1875. His father was Samuel.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_re…
Mary Shannon, spinster, of Ballymoghan died in the workhouse in 1889 aged 83. She presumably is part of the family.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_retu…