Macken/Rea and child emigration to Argentina and USA

I am researching my great grandmother Mary Macken (or Mackin), who was born in Ballynacargy about 1865. Her parents were William Macken (or Mackin) and Margaret Rea (or Wray or Ray), farmers from Slane Beg township. She had six sisters Elizabeth (born 1867), Frances (1869), Margaret (1874), Bridget (1876), Martha (1881), Kathleen (1886) and four brothers Patrick (1863), James (1870), Joseph (1872) and William (1879). Bridget, William, Martha and Kathleen were born in Slane Beg, the other children in Ballynacargy.

Sometime before 1889, Mary and her sister Elizabeth were sent to Argentina. They appear to have been unaccompanied and travelled “bound to the captain”. Both sisters married, raised families and died in Argentina. It is believed that two of the other sisters (Margaret and Bridget) were similarly sent to the USA. William and Kathleen also emigrated to the USA.

Any information on the Macken/Rea family would be of great interest. I am also interested in the circumstances around the emigration of the girls (why, how, when).