Carolyn and Mary - thank you both so much for your responses and for taking the time to look into this for me. It's so helpful to have fresh pairs of eyes on the case, and new avenues to explore.
I've revisited the information I have for Kate. The 1939 register gives her DoB as 2/4/1864. If correct, this would mean she was 19, not 20, when she married Wm Bubb on 27/9/1883. Not far out, but the fact it doesn't say 'Full age' suggests to me she was under 21 when she married (his age is given as 'Full'). So I'm inclined to think she was born around 1863/1864. Her youngest child (my GF) was born in Aug 1906, so if she was born in 1858, that would have made her 48 when she had my GF. Not impossible, but unlikely?
That said, the 1891 census has her as 32 (i.e. born 1859); the 1901 census has her as 39 (i.e. born 1862); the 1911 census has her as 45 (i.e. born 1866); and the 1921 census, taken on 19/6/1921, has her as 57 years and 2 months (i.e. born 1864). She died in March 1949, with her age given as 86 (i.e. born 1863). I'm inclined to go with the 1921, 1939 and death cert information, which ties in with her being born in 1863/1864 - but am happy to be persuaded otherwise!
As regards her mother (or possibly step-mother), the 1891 census shows Ann Driscoll ('Mother') living with Kate and her husband in England. It says she is married; and gives her age as 70 (i.e. born 1821)
The 1901 census also shows 'Annie Driscoll', 'Mother' living with the now widowed Kate and her family in England. But gives her age again as 70 (i.e. born 1831). Interestingly, Annie is now listed as being 'Widowed'. I've found a possible death cert for Annie Driscoll, which says she died in June 1908 aged 88 (i.e. born 1820) but has no info regarding a husband.
I'll revisit this all in the next day or so, but thank you both again for your suggestions. If you've stuck with me this far in this message, any further advice or ideas would be much appreciated!
Elaine
p.s. If it helps tie things down any further, Kate Driscoll's residence at time of her marriage in 1883 is Gubbeen, Schull.