Researching the Clarke family

My ggrandmother Margaret Clarke born in 1839 in Dundalk County Louth. She left Ireland and arrived in New Orleans La in 1851. I know that she had a brother named Henry who was in her wedding in New Orleans. She also had a cousin named William Clarke, who also settled in New Orleans. I have found an extract of a census from Ireland dated 1841 & 1851 that listed a Margaret Clarke from Barony Ardee, Parsh Kilsaran. Her parents listed as Michael and Bridget clarke living with Mrs Margaret Reid c/o John Reid on Hill Street Dundalk.Could this possible be my ggrandmother(she was Roman Catholic) and why was she compelled to leave Ireland at such an early age to travel across the sea to America by herself. Any info will be greatly appreciated. Joyce Murphy Dienglewicz