The marriage in Ireland was to avoid problems with marrying a deceased wife's sister - illegal until 1920s.
However if my match of Wm in 41 IoM census is correct and if his age at death is correct (and these are not always so) then there is no immediately obvious match in parish regs - also the initial spelling of his name is in itself strange as Taggart was a well established Manx name tho possibly less in Douglas than in South of Island and thus this strange spelling might well indicate a non-Manx background - the late 1790s saw disturbances in Ireland and the movement of a number of Irish to seek a quieter place to live on the Island - the Island economy was not then particularly strong but the Napoleonic wars saw a greater demand for local agriculture and Manx agriculture went thro a small boom period + drew in many labourers from Ireland.
Thus I wonder if the origins of Wm's family lie more in possibly Ulster than on Island