Hi KAJ58 and Chezz
I think from the info you have both recently posted, you are likely both talking about the same families. KAJ58, the death cert details for your grandmother certainly have the same details as Chezz has for her ggrandmother Elizabeth's family you would think.
Some further ideas for research:
You have the 1886 marriage for John McGuire and Margaret Harkin/McIlhenny with the relevant details.
You also have the entry (not sure if you have viewed the actual marriage cert) for a Margaret Harkin to a William McIlhenny. Both these certs (together with any relevant death certs) will let you confirm parents for Margaret. There is no confirmation for now that John McGuire was actually your grandmother Margaret's father, KAJ58, given the date of the marriage (1886).
Working through possible censuses. Can't really see anything for the McIlheny family for 1871. For 1881, there is this possible entry (a whole listing of names, I am putting a link together because of the surname):
Margaret McIlhemy, 29, hawker, prisoner, b. Greenock (marital status not showing on the transcript I am looking at)
Margt McIlhemy, 10 months, a prisoner's daughter b. Greenock
Address: 3 Nelson St. West Prison, Greenock West
There is a birth showing in 1880 in Greenock for a Margaret McIlhenny which likely fits with the above census entry? Details from this cert would help to confirm whether the right entry for young Margaret. What would also really help is confirmation of Margaret's birth place, from her 1911 census entry after her marriage.
There is also a death showing in 1879 in Greenock for a William McIlhenny, born c. 1846. KAJ58, you should be able to confirm it is the right entry from parents' details given for him and wife's name's etc.
For 1891, after the 1886 marriage and the birth of the two girls you have Chezz, including your ggrandmother:
John McGuire 41, Ship Yard Labourer
Margt McGuire 40
Ann McGuire 4
Elizth McGuire 1
Address: 59 Dalrymple St, Greeonock
No sign of young Margaret there in 1891, but a few possible entries for example for a Margaret McGuire in an orphanage in Glasgow, born in Greenock.
Monica