I can’t find a Samuel son of Samuel baptised c1830 in County Down. Perhaps the reason Samuel and Margaret left Ireland was because they wanted to marry but would have experienced some hostility where they lived because they were different denominations. I can’t find a baptism for Margaret either. Church of Ireland, Roman Catholic and Presbyterian records decrease in availability the further back you go. Some more information about Samuel, you might have this already:-

1881 IOM Census, 21 Market Street, Douglas, IOM, Samuel Stewart, Head, married, 50, car proprietor, Co. Down Ireland
- Servant, unmarried, Margaret Mason, 21, domestic servant, Douglas Isle of Man

Brown’s Directory 1881/82, under “Car Proprietors and Livery Stables”:-
Stewart, Samuel, 21 Cattle Market-street

1891 IOM Census, 6 Great Nelson Street, Douglas, IOM, Samuel Stewart, Head, widowed, 60, general labourer, Ireland
- Servant, unmarried, Margaret Mason, 22 [32?], domestic servant, Douglas Isle of Man

Burial
Name: Samuel Stewart
Age: 65 Years
Burial Date: 21 Oct 1896
Burial Place: Braddan, Isle of Man
Residence: Douglas