Looking for Information Regarding My 4x Great Grandparents

My 3x Great Grandfather is Thomas Henry. He was born 18 December, 1828 in Sheeptown, Newry, County Down, Ireland. He came to the US in 1846 and married Rachel Elizabeth McIntosh in 1854. We know nothing about his parents other that his father's name was also Thomas Henry and we believe his mother's name was Jane. That is all we know. We have been at a standstill genealogically for years because of this. Any information or suggestions on where we could look that we maybe haven't yet would be most appreciated.