Thomas Ross

Thomas Ross is a common name in Ireland. There were 91 in the 1901 Irish census and it would have been equally common in 1830. You will need to get Thomas’s parents names from say a marriage or death certificate in the US to try and narrow the search. 

A birth around 1830 is well before the start of birth registration in Ireland (1864) so you will be relying on church records for his baptism. Not all have survived and of those that do, not all are on-line. Without some idea of precisely where he came from it may be a difficult search.

Being Presbyterian tends to point to the counties of Ulster but even then it’s a large area. You can see where the surname was found in the mid 1800s using this link:

https://www.johngrenham.com/surnames/

It was most common in Co Antrim but was found in many other counties too. Wiht a surname like Ross and being Presbyterian points to the family having originated in Scotland, probably in the 1600s as that's when the majority of Scots settled in Ireland.