Thomas Carroll

I've been concentrating on my husband's Carroll family and there are so many questions unanswered.  When was Thomas born?  The only info I have is from his tombstone and his death certificate and that information came from one of his children.  Who were his parents?  His death certificate says his father was also named Thomas Carroll and his mother's name was unknown.  Thomas Carroll is the most common name in County Tipperary where Thomas was from according to his tombstone and death certificate.  Without a mother's name, where do I go to learn about his family?  When did he come to America?  Was he already married or did he meet Margaret Blackburn in the US?  Did he leave Ireland because of the potato famine? Or, did both his parents die leaving him to seek a new life on his own?  There was a Thomas Carroll arriving at every port on almost every ship on the eastern seaboard for years.  The passenger lists I've see do not list any Thomas Carroll with a Margaret Blackburn or Margaret Carroll.  Some Thomas Carroll's are accompanied by other Carroll family, but it's not known if our Thomas Carroll had any family come with him or if he was meeting someone here in the US.  The Griffiths Valuation lists only one Patric Blackburn in County Meath where Margaret was born and he lived in the city so he apparently was not a farmer.  I can not prove that he is her father.  The only name given for her mother on the death certificate was "Marie".  Thomas and Margeret are my brick wall for this family.  I can easily follow them from the time of the 1860 Federal Census, but not before.  There are Thomas Carrolls and Margaret Blackburns in the 1850 Federal Census, but I can not prove any relationship to my Carroll family.  If anyone, anywhere has a suggestion, I will gladly follow it.  There are so many unanswered questions.