Did you "come across" this will at: www.iomfhs.im/lawsons/twill/1843_024.html?
If so, please acknowledge the source, and the person - Lois Ralph - who went to the trouble of transcribing it and making it available to others.

There is an 85-yr-old Isabella Kelly in Douglas on Brian Lawson's census index:
www.iomfhs.im/lawsons/LawsonBMD/census/1841-census/c41-k.html
[1841 census rounded adult ages, and no relationships given.]
Household Members: Name Age
John Kelly 55 joiner
Ann Kelly 50
Isabela Kelly 85
Jane Kelly 20
Margt Kelly 16
Ellen Kelly 15
Robt Kelly 11
Will Garret 25 lab

William Taggart, 45, shoemaker (as given in the will), is also in Douglas, at James St., with Isabella 35 and (assumed) children Eliza 13, James 11, Isabella 10, Wm 2, and also Mary? Taggart 75.
All the above children's bapts are at Saint Mathews, Douglas [IGI], plus James bapt 12 May 1844. Also John bapt 1837, missing from the census.

The IGI also has children of James Kelly & Isabella Kermod whose names match those in the will, all b.Braddan. Son John bapt 1783; Robt bapt 1792; Margaret [Taggart?] bapt 1790; and Cath [Millburn?] bapt 1795.

In the 1851 Douglas census, at James St., no.7: [2525 204]
Household Members: Name Age
William Taggart 54 boot & shoemaker b.Onchan
Isabella Taggart wife 49 b.Braddan
James Taggart son 21 boot & shoemaker b.Douglas
William Taggart son 11 scholar b.Douglas
Robert Taggart son 6 scholar b.Douglas
William Kelly lodger U 50 joiner b.Lonan
William Cubbon lodger mar 30 joiner b.Castletown
Jno Preston lodger U 28 blacksmith b.Castletown

Is your husband's Taggart grandparent there?

Sue