I'm glad you both think it possible that someone could appear twice in the census, as the alternative might have been a whole new family that I didn't know about, and whereas missing two Thomas Cavendishes off the IGI is curious, three would be very odd! Jurby Thomas was on Microfilm 106725 (baptisms 1840-1871 St Patrick, Jurby) – [MNHL May 05]
1845, Sept 21st (No. 176) Jurby, St Patrick
Thomas Cavendish, s. of Ann Curphey, Bally Meanagh, Shoemaker, [father’s name omitted]
I didn't record the film number of the Kirk Michael register film, but the details are as shown in my original posting.

Out of interest, these weren't the only names from my family to turn up in the registers and not be in the IGI, especially for Kirk Michael. It would be interesting to know if other researchers have found the same thing with their names, and if this is common.