Catherine Collins and Daniel Curtin

My great grandparents, Catherine Collins from Brosna, Knockbrack? b circa 1834 and Daniel Curtin b circa 1825 Castleisland, immigrated to USA and settled in Washington, DC, where Daniel worked as a dairyman/milk peddler and served in the Union army in the American Civil War. I believe Catherine had a brother, William Collins, who also immigrated and settled in Washington, DC. I am looking for any additional information on them and their families, their immigration, as well as any descendants in Ireland. I have not yet been successful in finding birth, baptism or other records in Ireland.