in the 1851 Welsh census , are two James Mountain are either your James ? but I am having trouble with my computer this pm, I doubt if either james is the right person. Searching for a Thomas Mountain came up with John Mountains family but it might have been badly reported in whatever you have read. I got this census off my Genes Reunited membership.
1. living in the hamlet of Ebbw Vale New Colliers Row Bedwellty Monmouthshire is John Mountain a Rail Straightener also within the iron worksb 1819 from Waterford with his wife Ann b 1818 Cork and children inc a james Mountain aged 9 ie b ~1842 in Waterford, together with a cousin from Cork one Johanah Mountain aged 22 b 1829 co Cork together with a brother Michael.............. and a brother William b~1829 Waterford. Johns father Michael b 1796 Waterford is also living there. In 1861 the same family live in Pembroke Street Aberdare in the Parliamentary Borough Merthyr Tydfil
The above family includes a Thomas b1850-1b Bedwellty
2. The second James is slightly older, son of Patt and Mary Mountain from Ireland lived in Wales before the 1841 census having at that stage a James Mountain born in ~1838. In 1851 they were living in Chapel Street
Merthyer Tydfil Glamorganshire But I couldnt find them in the 1841 census. James senior and most of the other occupants of the area were Rail men. I believe this is within the Iron works.
Sorry thats all I can offer JimG