Kathleen,
Charles & John O’Neill were listed in 1827 as farming in Dirnan in the tithe applotment records:
http://www.irishgenealogyhub.com/tyrone/tithe-applotment-books/parish-o…
Griffiths Valuation for 1859 lists Sarah O’Neill in Dirnan. That suggests Charles was already dead and she was a widow. She had plots 19 to 22 which were a total of about 14 acres. Next door on 23 & 24 was Mary O’Neill (probably another widow, presumably of John O’Neill) with 5 acres. Those farms today are on the modern Dirnan Rd. Charles and John were probably related (brothers perhaps). The Valuation revision records show Sarah’s farm changing to Peter in 1882. Following Peter’s death in 1914 the farm changes to James in 1918 and then to Joseph in 1920.
Death registration didn’t start in Ireland till 1864 so if Charles died before that as seems highly likely there may be no record, unless he has a gravestone.
Still 2 O’Neill farms there in 1901:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Lisson_Upper/Dirnan/1533897/
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Lisson_Upper/Dirnan/1533894/
That second couple were Charles O’Neill and Susan Holly. Charles’s father was Michael O’Neill.
Probate abstracts from the PRONI wills site:
O'Neill Charles of Dirnan county Londonderry farmer died 22 April 1901 Administration Belfast 7 December to James O'Neill farmer. Effects £48.
Probate of the Will of Peter O'Neill late of Dirnan County Londonderry Farmer who died 17 February 1914 granted at Londonderry to Matthew Conlon and Joseph O'Neill Farmers.
O'Neill Michael of Dirnan Lissan county Londonderry farmer died 22 May 1948 Probate Londonderry 15 July to Charles O'Neill farmer. Effects £725 19s. 1d.
O'Neill Joseph of Dirnan Lissan county Londonderry farmer died 30 May 1954 Probate Londonderry 19 August to James O'Neill and Patrick Donaghy farmers. Effects £891 11s. 9d.
Sarah O’Neill death in 1877 aged 86. Informant was Kate O’Neill.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1877/020551/7211912.pdf
Lissan RC’s baptism & marriage records don’t start till 1822 so you won’t find a record of Charles or Sarah’s baptisms or their marriage, unless it was in 1822/23.
I had a look for marriages for Peter’s siblings but the surname is so common in that area, I could not be sure of identifying any. So I have no idea where they went, except that his sister Mary evidently never married as she was still on the family farm in 1911:
http://www.census.nationalarchives.ie/pages/1911/Londonderry/Lissan_Upper/Dirnan/614591/
She died 12th May 1912:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1912/05368/4497143.pdf
There’s an RC graveyard in Broughderg, parish of Lissan. That might be where the family are buried.
Elwyn