A quick update on this as I've put together a few more pieces of evidence.
Firstly, I've found another possible child of John Tomlinson senior. Matilda Tomlinson married a Samuel Doyle at the RC chapel of Moneymore in 1873. She was listed as living in Ballyronan, Samuel in Ballymaguigan. The couple can be found in the 1901 census still living with a large family, still in Ballymaguigan, with Samuel and some of his sons working as Fishermen.
Secondly, I took a look at the griffiths valuation books to see if I could find any useful information about the Tomlinsons in there. There's a leased plot of land of about ten acres with two houses on it in Belagherty. The first house is rented by a William Tomlinson, the second by a Stephen Tomlinson.
I'm fairly certain this is the same protestant Tomlinson family I've already found in the records. William's portion of the land changes over to an Elizabeth Tomlinson sometime between 1860-63, with William presumably dying just outside the creation of civil registration. Elizabeth (or 'Eliza') dies in 1872 aged 65 with her land going to a John Woods. I believe this is the same John Woods who marries Sarah Tomlinson in 1861. I haven't found documentary evidence for this yet, but I believe that Elizabeth Tomlinson may be the wife of William Tomlinson, making her John Woods' mother-in-law.
Stephen Tomlinson also seems to be fairly clearly linked to this same family, as when he dies in 1872 at the age of 85 his land passes to a James Howard. I believe this James Howard is the son of John Howard and Mary Ann Tomlinson, another of William's daughters. Now Stephen also has another parcel of land; a cottier's house and land in Ballyneill. This also goes to James Howard on Stephen's death, but with all three of John Tomlinson's (presumed) children were resident in Ballyneill at the time of their marriages, it makes me wonder if there was some kind of connection.
As for John Tomlinson senior himself, I've found a listing in the Griffiths record in Ballynenagh for a John Tumilson (sic). He is renting a house from a Francis Marshall who seems to be renting out a series of houses on his almost 50 acres of land. The revision books show this land passing from Francis to a Mary Marshall (I presume his wife), then later to a John Marshall. John Tumilson's house passes back to John Marshall in the 1880s, which matches with John Tomlinson's death record, with him passing in Magherafelt workhouse in 1885.
So the two 'strands' of Tomlinsons in the area seems to be a little clearer, although I'm no closer on working out the mystery of the two Margarets. At the moment, I can only see the one candidate for her (their?) husband Thomas Tomlinson. Although it does seem strange to me that, assuming the John in Ballynenagh is the father) that all the children are living away from him before marriage.
