Griffiths Valuation for Kilcootry was compiled around 1860, so if your Rosanna left Ireland c 1850, she’s unlikely to still be listed in 1860 and beyond. The one in Kilcootry was head of household, so probably a widow. The Patrick living nearby is most unlikely to be her husband. He could well be a relation by marriage but they could not be husband & wife.
You can use the Valuation revision records on the PRONI site to take Griffiths records forward.
https://www.nidirect.gov.uk/services/searching-valuation-revision-books
Those records show that Patrick had plot 11 which was a 9 acre farm. It remains in his name till 1880 when the tenant changes to John (who appears to be his son). Patrick Mullin died 17.2.1879 aged 75 at Kilcootry. His wife was still alive and his son John was the informant.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1879/06527/4880972.pdf
Rosanna was next door on plot 12. She had 3 acres of land but no house, so she was lodging with someone else in the townland. (There are several Mullin households there as you may have observed.) Her name is deleted in 1876 and her land appears to have then been annexed into one of the adjacent small farms. So she might have died around that time, or left the area. I cannot find a death for Rosanna but she would have been either a single woman or a widow. Until the Married Woman’s Property Act in 1882, a married woman’s assets were all her husband's and so she wouldn’t be a leaseholder in her own right. Indeed couldn't be a leaseholder.
I have known people who have emigrated retain land in Ireland, but usually it’s sublet and the new tenant’s name appears in the records. In most cases, if it is in someone's name in Griffiths, they were living there.
Donacavey RC parish records only start in 1857, so if that’s where your Rosanna was born or married, then sadly there are no records to consult.
Mullin/Mullan etc in Kilcootry in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Seskinore/Kilcootry/1743732/
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Seskinore/Kilcootry/1743733/
The above are children of Bernard Mullin and Mary Anne McGrale
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Seskinore/Kilcootry/1743735/
You can use this site to see the various cemeteries in Donacavey. There are 5 listed:
https://www.ancestryireland.com/family-records/graveyards-in-ulster/