Had a look at Griffith Valuation for Ireland 1846-1864 and surprisingly there are only 4 on property in all of Ireland. They are all in Tipperary South Riding (1850). In 3 Parish's Ardmayle, Ballysheehan & Gaile,
all are in the Poor Law Union of Cashel, no more than 12 mi. radius. (Co. Sligo is quite aways from Co. Tipperary N.riding)
However, both Sligo and Tipperary have their parish records on line, depending of course on the survival of parish records. It is a fee based but not for profit Irish History Foundation. You have a name that is unusual even if Margaret is common, You can search without a county. You must have a birth year or approximate one. I think it would be worth finding out. A parish birth record will give you mother and father's names and mother's maiden names, parish/place and
Few other Curwin's 1795 Dublin Grant Book - Jane Curwinwww.genealogy.nationalarchives.ie
Martin Curwin, Parish of Finglas, Townland Artaine N. Co. Dublin 1833.
1831 Tithe Defaulter - spelling Carwin in Co. Lough
It could be a matter of spelling? Ireland is famous for incorrect interpretations as most people couldn't write so who ever was keeping records spelled by the sound of the name. It was illegal at one time in Ireland to provide an education to an Irish catholic.
These are two UK census that identify born in Ireland persons.
The 1841 UK Census shows spellings of 16 Curwin, Carwin, Corwin, Karwin and Kurwin all born in Ireland
1871 UK Census same spellings and 17 born in Ireland.
Closest residence to Manchester was Liverpool and only 1 Margaret in the 1871 spelled Kurwin living in Birmingham, she and husband John are 53.