Ifv you look more closely at the KWC records (on manxnotebook) you will see Dr Cheyne moves from Castletown to Dublin around 1840-42, there are two marriages of Cheyne girls both pre 1841 census - Selina to Charles Graves (? father of A P Graves) + cecilia to Legh Richmond - neither appears to be in 1841 census so my guess is that the Dr moved to Dublin around time the two daughters married off at Castletown sep 1840 + apr 1841)