1) Sadly my details for Elizabeth Waddell remain sketchy - born 1827 in 'County Down' - parents James Waddell and Mary Kirkwood - exact date of arrival unknown, but first detectable in the 1851 census as Betsy Waddell, spinner in sail factory at Tromode Green. She then somehow moves to Foxdale where she marries Richard Callin on 10 August 1851. Although she eventually died 8 April 1903, I cannot find her in the 1901 census. The info I have on the Irish connection is 'word of mouth' and not verified. She might have come to the IOM with the Redpaths.
2) Decimus Powney was born in 1872, son of William Powney and Elizabeth Perrett, and married Alice Figgins in Devizes - one child, Dorothy Elizabeth Powney is listed in the 1901 census - there may be more later, of course. Decimus was the youngest of 18! He wasn't the tenth son born, but when he was born, he had 9 surviving older brothers. I obtained most info from an Ellen Harding back in 2001.
I hope this helps you find something, on either side. If you would like to discuss further outside the Bulletin Board, my email is email@example.com.