This report from the National Inventory of Architectural Heritage sheds some new light: "Griffith's Valuation of 1857 records that a William Armstrong was the miller here at this time, when the holding was described as consisting of a 'house and land, flour mill, house, office and yard' with a rateable value of £98 and 10s, and a 'house, corn mill and kiln' with a rateable value of £38 and 15s. This house was probably later the home of an Isabella Armstrong (Mill View House, Bridgetown) in 1894 (Slater's Directory)"
Below is a close-up of a probate record we found through familysearch.org. This confirms Isabella a 'spinster' - left hand column, second name down - date of death and she lived in Middlesex, LONDON. The gentleman mentioned, we believe, is a McKinney nephew.
Armstrong Grave in Laghy: Headstone transcription
Sacred to the memory of William Armstrong of Millview Bridgetown who died 10th Dec. 1874 aged 57 years.
His wife Anne who died
16 Nov.1891 aged 72 years.
Their son Mervyn
who died 11 Sept. 1888 aged 27 years.
Their daughter Isabella who died 26 Oct. 1927 aged 80 years.