Did siblings marry siblings? Yes - both had father Thomas Dooey, weaver, from Letterlone. A Thomas Dooey had a house and small garden in the Griffiths Valuation. Mentioned thrice in 2 Dunboe Baps.
A Martha McKeown is a widow in the 1901 census, living with 28 year old unmarried son, Thomas James.
1901 Census:
Residents of a house 4 in Letterloan (Letterloan, Londonderry)
Surname Forename Age Sex Relation to head Religion Birthplace Occupation Literacy Irish Language Marital Status Specified Illnesses
Mc Keown Martha 53 Female Head of Family Presbyterian Co Derry Farmer's Wife Read and write - Widow -
Mc Keown Thomas James 28 Male Son Presbyterian Co Derry Farmer's Son Read and write - Not Married -
She is 64 by the time of the 1911 census, a farmer, living with her single son, Thomas James, a farm labourer, aged 39.
1911 Census:
Residents of a house 8 in Letterloan (Letterloan, Londonderry)
Surname Forename Age Sex Relation to head Religion Birthplace Occupation Literacy Irish Language Marital Status Specified Illnesses Years Married Children Born Children Living
Mc Keown Martha 64 Female Head of Family Presbyterian Co Derry Farmer Read and write - Widow - - - -
Mc Keown Thomas James 39 Male Son Presbyterian Co Derry Farm Labourer Read and write - Single - -
The rest of her family just vanishes.