Just a quick review of children to David Donaldson and Mary Jane Long who married on 3 April 1888:
(1) Martha Donaldson born on 1 June 1889 (family address = 15 Crumlin Street, Belfast):
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1889/02466/1915939.pdf
(2) Unnamed male Donaldson born on 14 November 1890 (family address = 15 Crumlin Street, Belfast):
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1890/02411/1898952.pdf
I've really struggled with tracking back Mary Jane Long, the only information about her background was her father's details in that 1888 record of marriage to David Donaldson - namely William Long, a coachman, deceased at the time of that marriage. As I said at Reply #21, the St. Peter's version of that marriage record elaborated on that to identify her mother as well - Mary Anne Stewart.
I couldn't find a William Long - Mary Anne Stewart marriage, nor could I find a birth/baptismal record for Mary Jane Long.
But then I happened upon the following baptismal record for a John Long, he was baptised in Glenavy & Killead RC parish on 4 September 1853, the baptismal record shows his parents as William Long and Mary Ann Stewart, William was recorded as 'Prot' (Protestant), son John was identified as 'illeg.' (illegitimate) and the sponsors were Bernard & Rose Jane Stewart (just over halfway down left page in the following link):
https://registers.nli.ie/registers/vtls000633183#page/10/mode/1upIn tracking John Long forward, the following marriage registration jumped out, it took place in St. Patrick's Belfast about 3 years before David Donaldson and Mary Jane Long got married, it shows a 30 year old John Long marrying an Annie McCune, John's father recorded as... one 'William Long Deceased', 'Coachman':
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1885/10865/5967629.pdfJohn Long and Annie McCune went on to have a number of children, the birth that is most interesting is of their daughter Rebecca Long, born on 15 June 1891. Previously, at Reply #24, I listed the children born to David Donaldson and Mary Jane Long, the first two, Martha and an unnamed male (David?) were born in June 1889 and November 1890 respectively, the family address for both was 15 Crumlin Street. The family address for Rebecca Long, born 7 months after that unnamed Donaldson son, was... 15 Crumlin Street:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1891/02381/1888455.pdf So notwithstanding the lack of a birth/baptismal record for Mary Jane, a hypothesis at this point has to be that she and John were maybe related, maybe siblings, and that maybe therefore she too was the illegitimate child of that protestant coachman William Long and Mary Anne Stewart.