Hi, my great great grandmother was Margaret Dempsey whose mother's maiden name was Cahill from Co. Roscommon (Castlerea/ Lisleddy/Emlagh/Ballintober area I guess, as her husband Martin Rourke was from around that area). I haven't done any research on her but years ago, I found the following information but I unfortunately did not record the source.
Mgt was born on 5/7/1835 to John Dempsey and Bridget Cahill. Her godparents were Thomas Egan and Mgt Cahill. Her siblings were John 5/5/1828; Mary 7/6/1833; Catherine 28/11/1840 and Ellen.
Mgt's sister, Ellen Dempsey emigrated to America (New Jersey) and years later three of her nieces followed her and gave her address as they entered at Ellis island.
Mgt married Martin Rourke in 1856 so those three nieces were Roarkes and Bridget Cahill's grandchildren. There are many descendents of these 3 nieces living in America today.
I hope this may be helpful to you.
Good luck with your search
Marcie