RIC policy was not to post a police officer to a county he or his wife had family connections. So Gilbert is unlikely to be a native of Cork and you would expect the family to be transferred to another county after the marriage.
I see children born as follows on rootsireland:
Dora 20.4.1877
William 20.4.1879
Elizabeth 6.6.1880
Joseph 15.3.1882
Sarah 15.4.1883
All the above at Castlecomer, Kilkenny where Gilbert was Head Constable (a rank between sergeant and Inspector).
Then an un-named male 16.3.1886
Edith 19.9.1889
Both the above were in Tramore Waterford. Gilbert’s occupation then was railway clerk.
According to the 1901 census Sarah was born in Cork East Riding c 1850.
http://www.census.nationalarchives.ie/pages/1901/Waterford/Tramore/Patr…
She married in a Wesleyan chapel (though I notice that by 1901 she’s Church of Ireland). Tradition was to marry in the bride’s church, so I would try and find out if that chapel has baptisms for c 1850. (Many Methodists at that time still baptised children in the Church of Ireland). If you can’t locate any Methodist baptisms then it might be worth trying the local Church of Ireland for the parish of Monkstown. So I’d be inclined to search the parish records for the parish where she married and perhaps some adjacent ones, to see if you can identify her birth.