Having Trouble Tracing My Original Irish Ggggrandparents

I cannot find the name of the ship they came over on, or where exactly where they are from. They arrived in America in 1847, and any record I can find says Sligo as their origin. But I have found out that it was actually the last port. How do I find them? Their name was Ginty, or Guinty, or Mcginty. The parents died of Cholera in the early 1850's, and their youngest child was born in Illinois, 1849? Their names were Patrick and Mary Ginty, Mary's maiden name was Hanley. The oldest record I have for them was an 1850 census record in LaSalle, Ill. Patrick is listed as age 40, born in Ireland. Mary is listed as 35, also born in Ireland. I don't subscribe to any pay sites, so I have limited access to online records. I would really appreciate the help!