Because of various committments, I haven't been able to get to the Ref Library- so I haven't ben able to check the 1821 and 1822 Port Dalrymple Musters, for John mcNally or the Cummings household. But the following has turned up online in the Archives Office "Departures" index
'John MacNally, seaman, 28 January 1818, from Hobart,per" Governor Macquarie", to Port Dalrymple and Port Jackson"
OK- is this our man? and what are the circumstances? and was he just going to Port Dalrymple or right through ( leaving) to Port Jackson? Last first- if he is our man , then he was in the October 1819 PD Muster so he wasn't leaving the island in 1818. Is he our man- our suspect seems to have crewed on a number of sealing ships before being employed by Cummings in 1816, so " seaman" is good, the name is right, and the date is OK. The circumstances? Anne Cummings has left the nasty Captain Cummings, and in 1819 he is still advertising for her and saying she's been gone some years, well, lets say a couple, and we have surmised that Catherine McNally her servant went with her- so was husband John McNally looking for her, did he find her, told to get lost, or didn't find her, but in any event is returning to work and the children at Port Dalrymple ?
And.... if he had found her, did he try for a reconciliation again,say in 1820(by which time Anne Cummings had returned to Captain Cummings, but Catherine was still in the household of Thomas Ransom) ,and the result of the attemped but unsuccessful reconciliation was Thomas McNally1820, which might indicate an error of judgement by Catherine , but at least he was her legal husband and it eliminates the taint of promiscuity from Wiggy's Great.
Different thought- to release Catherine to remarry again in 1830 , John Mcnally has to die /disappear between 1825-1830, so is the death in 1825 of the by now completely gaga Captain Cummings a significant factor? If McNally was still part of the Cummings household, did he then have to move on?
As a footnote, forget all about Catharine Rook , even if she wasn't massacred in 1809, because she was Catherine Rourke per "Sugar Cane" 1793, and was born c 1772, making her a touch old to be our Catherine producing young Thomas in 1820.
David ( but call me Obadiah, or was it Ishmael?) Moby Dick..Whales,seals... its all too much !