Hi Jillray
It's well accepted that research in Ireland can be very difficult at the best of times, unless you have a distinguished, or otherwise noted family, a unique name, or know the townland, or names/occupations of relatives, have some more precise dates, some death or burial details and so on- but Sarah HARRIS b Cork abt 1870 with a sister named "Kate" is unfortunately - not in that category. You
are fortunate at least to have "Cork" as a pob. and not just Ireland (as many of us have, me included

)
I would suggest that you find out more about Sarah - and Kate - after they came to England as it may be your best chance of locating them in Ireland.
I suppose there is also the possibility she was born to English parents, where the father was in the Army and stationed in Ireland for a time.
Were the sisters in England in time for any of the Census 1871 to 1911?
Since she is your grandmother, I presume you know who she married, at least? You can search for a marriage on FreeBMD:
http://freebmd.rootsweb.com/Cheers
AMBLY