In response to Amy's contribution:
"I had also concluded that John McGuire was married to a Sarah with a child, Margaret (born in South Australia). Can confirm that Margaret is a step-daughter of Mary Grant, as Mary's newspaper obituary mentions a stepchild, (Mrs M. Johnson). As mentioned above, Margaret McGuire married Walter Joseph Johnson."
Standard research principles call for primary evidence, and secondary evidence wherever possible. Such principles always rank higher than 'conclusion' and 'contributed information' found in Obituaries, albeit from family members, this amounts to heresay until supported by primary evidence.
Is there evidence of a child, Margaret, born locally? If so, reference the SA BDM record as your base evidence.
Supportable Facts Sumary
Mrs Walter J. Johnson, married after the 27 Jun 1885 death at Adelaide Hospital of husband John McGuire, carpenter from Omagh Co. Tyrone, arrived 23 Jul 1875, aboard Hesperides. Ref: SA Archive Shipping Lists and Adelaide Hosp. records.
John McGuire carpenter, son of Daniel (carpenter) and Margaret LYNCH, dau.of William, married at St Laurence Church, North Adelaide 01 Nov 1875. Ref: Book/ folio: 105/321
Over the period 1875 – 1885, 7 children born to Margaret (nee LYNCH) & John McGuire, occupation Carpenter stated in Birth Certificates, being 3 sons and 4 daughters. No daughters survived infancy. The last born 1885, after death of John, was the subject of an inquest: Finding suffocation by pillow.
The inquest revealed concerns about previous infant-deaths, mother's drinking habits and disparity between older brothers' statements to police and that sworn by the mother. Ref: Inquest file; SA Archives @ Gepps Cross.
15 Nov.1886 Margaret (widow) McGuire remarries Walter J Johnson, as Margaret McGuire, rather than as LYNCH. By common practice a woman marries under her last known name. Johnson, a bushman from Wilmington, 20 years her senior.
Of the 3 McGuire-Lynch sons, 1876: William (Will), 1877: John and 1880: Daniel Peter, the youngest died 1913 @Wilmington Ref: 378/413. Unconfirmed, but by word of mouth from a Johnson descendant, Will & John never married.
Death Cert. of Widow Johnson, 29 Mar 1915, states "Birthplace & Length of residence in Australia: Ireland. 40 years in Australia" and "Age at Marriage or Remarriage: Remarried 30 years" Ref: Book/folio 392/79
From the Death. Cert. information, it can be inferred that Miss Margaret LYNCH entered Australia around 1875. However, as about 20% of S.A.Shipping Records are not extant, naming the vessel is problematic.
DNA as supporting evidence
To test the hypothesis that a baby born 1855 in S.A. to a Sarah and a John McGuire is the same person as Widow Johnson (d. 1915), descendants of the said John McGuire and descendants of Mrs Johnson could avail themselves of DNA testing.