The 1861, 1871, 1881 & 1891 censuses were all taken in April of those years, so her reported age of 34, 44, 54 & 64, respectively, is consistent with birth in May, 1826. And in 1851, she had just turned 15 the month before the census, so that fits also.

In 1891 she was listed as a “visitor” (and a widow) in the household of Thomas Starmer, age 53, in Hastings, Sussex (present East Sussex). Also on this occasion, she is described as “Living on own means.” Relationship of Thomas is unknown; possibly a cousin or nephew. Seems unlikely that he was a brother. [804] It could be the Thomas Starmer, born 1-Feb-1838, son of John Starmer and Mary Ann Chamberlain. [823] John & Mary Ann were married 05-DEC-1831 at Northampton St. Sepulchre C of E. [479]

At the time of her death, Mary was living at the Plas y Coed Hotel in Colwyn Bay, Denbigshire, Wales. I'm not sure if it was actually called a hotel at the time (1900); it was later known as the Plas y Coed Methodist Guild Guest House. The building was demolished circa 2002-2003. It is not known why she was in Wales.

When Mary’s will was probated, 03-OCT-1900, her estate was valued at £2857 7s. 10d. [8127], [8128] According to www.in2013dollars.com/UK-inflation, that amount would be worth about £353,474.05 ($477,189.97) in 2020.

In her will, Mary left £5 each to: Baptist Foreign Missions, The Zenana Missions and the College Street Band of Hope. £10 was left to her niece, Ann MAIN (relationship as yet to be determined; so far it doesn’t seem possible she could have had a niece with the surname MAIN). The will reveals that she was in posession of two large portraits of herself and her late husband, which she left to her son, William Starmer SMITH, but if he didn’t want them they were to go to her daughter, Kate SMITH HALL. She also had “family Portraits and views of his [William Starmer SMITH] house and ground in British Columbia, which also were left to William. There was a set of alabaster figures left to daughter Kate HALL and son-in-law W. E. PEBODY [“Edward William PEBARDY ” in this family tree]. Kate also inherited Mary’s watch, silver and plated goods, jewelry, linen and wearing apparel. All other personal estate and real estate was to be sold and converted to money by her her Trustees and Executors, and after expenses, divided into four parts: one each to son William and daughter Kate (or their children). If William, specifically, died before Mary, his 1/4 was to be given to his wife and children. The other two quarter shares were to be invested, and the interest or income to be used for the maintenance, education, apprenticeship or otherwise for the benefit of the children of her late son, John Alfred SMITH and the children of her late daughter, Sarah SMITH PEBODY, until they reach the age of 21. Upon turning 21, those children were to be given their "presumptive shares." If any of those grandchildren died before the age of 21, their share would be added to the remainder for the others. Mary appointed her friends, William Clayson RUSH and Richard TIMMS, as Trustees and Executors of her estate and left payment to them in the amount of £40 each.
[8128]

The district Conway spans the boundaries of the counties of Caernarvonshire and Denbighshire, but Llandrillo yn Rhos, was within Denbighshire.

“Billing Road Cemetery
Mary Smith aged 74, of Plasy Coed, Colwyn Bay, Llandrillo yn, Wales died 19 05 1900 buried in same grave as Thomas her husband No. 12130”
— from “Sandy Hall” on Facebook.
The Northamptonshire Record Office have the cemetery records on microfilm.
“The Northampton County Council Cemetery Office have them too.
No records are held at any of the actual cemeteries in the town.”

From wikipedia: “Llandrillo yn Rhos is the name of an electoral ward in Conwy County Borough, Wales.”