there are NO emigration records taken either by Manx or UK authorities - they would almost certainly travelled to Liverpool for passage on one of the many boats travelling to North America - often if you can't find on the USA manifest it means they may have travelled via Canada (this it seems peaked c. 1847 due to changes in US immigration law;
There may be incidental notices eg in published family letters, wills or even newspaper account (www.manxnotebook.com has a listing of those held by Manx museum + known to me in about 10 years ago.)