Thank you so much for all those replies, which have helped fill in that section of family history for me. The original death announcement, from the Belfast Newsletter of 7 December 1897 read 'M'CULLOCH - December 6, at Nedeen, Coleraine, Mary Crosbie, widow of the late William M'Culloch, Bangor, County Down, formerly of Dalmellington Ayrshire. Funeral private. No flowers'
William died in Bangor in 1883, and was indeed a builder, but no trace of Mary, until now. Obviously she moved to Coleraine because Agnes was there, which I didn't know.
Now the dots are all joining up. It looks as though Agnes retired around 1915 (she would have been 60), had a furniture sale in June 1914 at Nedeen, and at some point moved to join her sister in Dorking, Surrey, and died at Hove, Sussex in May 1936 - and I've now found a death announcement in the Belfast Newsletter of 21 May 1936 which confirms that she was 'formerly for many years Headmistress of the Irish Society's Girls' School, Coleraine'
So thanks for all your responses and links. Much appreciated.