All indications are that the children of Isabella McGlynn were illegitimate.
Mary & Isabella born 1887- no father listed.
https://www.irishgenealogy.ie/files/civil/birth_returns/births_1887/02541/1940717.pdfAnnie born 1890- no father listed.
https://www.irishgenealogy.ie/files/civil/birth_returns/births_1890/02433/1905917.pdfMargaret born 1898- no father listed.
https://www.irishgenealogy.ie/files/civil/birth_returns/births_1898/02076/1792449.pdfIt wasn't uncommon for illegitimate children to list a stepfather/mother's partner, other relative or even make up a name for the marriage registration. Since their surname was McGlynn that would be listed as father's surname, John may be the Christian name of their father but no way to be sure at the moment.
Annie McGlynn married 1913 Bruce Wallace Morton and they had several children but it is the birth registration of son Bruce Thomas Morton in 1919 that is interesting- the informant is Margaret McGlynn of Whitehouse-
https://www.irishgenealogy.ie/files/civil/birth_returns/births_1919/01250/1519545.pdfSo, this connections Margaret McGlynn to Isabella, Mary and Annie. I believe Agnes is another sister although I haven't been able to find a birth registration.
https://www.census.nationalarchives.ie/pages/1901/Antrim/Whitehouse/Lr__Whitehouse_Street/988400/ (boarder Agnes McGlinn, 6, Co.Antrim, RC, scholar)
https://www.census.nationalarchives.ie/pages/1911/Antrim/Whitehouse/Cahoon_s_Street/191182/ (boarder Aggie McGlynn, 18, Belfast City, RC, linen spreader)
Now to an interesting bit.
In 1920, Mgt had a daughter in Belfast that died at 2 months, Mgt Mary. Per cert, baby's aunt was stated to be Isabella Cullinan.
https://www.irishgenealogy.ie/files/civil/birth_returns/births_1920/01210/1503472.pdf (father not listed)
https://www.irishgenealogy.ie/files/civil/deaths_returns/deaths_1920/05113/4405934.pdf (Isabella Cullinan aunt)
We know that Margaret married John Patton in 1922 but he turns out to be the father of baby Margaret Mary!
Northern Whig, 19 Nov.1920: BREACH OF PROMISE. Belfast Case Decided.... of the County Courthouse to hear several actions remitted by the Lord Chief Justice. The first was a case brought by Margaret M'Glynn, a minor, by her sister and next friend, labella Cullinen, 21, Bell Square, Lower Whitehouse, who claimed damages for alleged breach of promise against John Patton, Barbour Street, for damages for the seduction of her sister and servant, Margret M’Glynn. The jury awarded £100 damages ...
[not sure of exact order since I can only tease out snippets from free searches] paid her every _, but a regrettable feature was that misconduct took place in the month of _ last year. A child was born in July of this year, and died last September. Plaintiff was examined, and stated that she understood that the defendant was earning £2 15s a week at present. He had to serve another year as an improver. ...
Will let you digest this (assuming you haven't already found this information) before posting a bit more information about the mother Isabella McGlynn's family.