Hi Lynn You would need to know the address in County Meath where Michael Reilly lived in order to narrow down the search. I note the book entitled "The History of Will County, Illinois", states that- 'FRANK T. O'REILLY, farmer; P.O. Braidwood; was born in Meath Co., Ireland, May 1820, and is the son of Michael and Rose (Tully) O'Reilly, of Ireland; his father was a blacksmith and a farmer; was raised on his father's farm; at the age of 14 years, he was engaged in driving a dray between Kells and Dublin, and engaged at this business until he emigrated to America, in 1847, with his mother and children; his father died when he was about 9 years of age'.
Have you found the tombstone of Frank in Illinois and does it state where he was born? As he worked in Kells it is likely that he came from that parish. If you look at the records of the Meath Genealogy Project (online at http://www.rootsweb.ancestry.com/~irlmea/http://www.rootsweb.ancestry.com/~irlmea/) you might be able to find something in the Kells parish records. Failing that you could contact the Meath Heritage Centre (online at http://meath.rootsireland.ie/http://meath.rootsireland.ie/) who should be able to help you.