I'm Argentine and currently I live in Barcelona (Spain). I'm the great-great-gradson of John and Kate.
Facts I know about John and Kate according the docs I have:
1. John McDonnell was born in Ireland between 1829 and 1834, Kate Duffy also was born in Ireland between 1834 and 1836 2. According the passenger arrival list of Buenos Aires (Argentina) on September 9, 1857 arrived in the same ship John McDonnell, Catherine McDonnell and William Duffy. Their names are in 3 consecutive lines. 3. According the parish baptism record of San Andrés de Giles (Argentina) on May 5, 1858 was born he first “legitimate” daughter of John and Kate named Maria. The godfather was James Duffy. 4. According a baptism and marriage data base of the Ardagh parish (Co. Longford) William Duffy and Mary Shanly married in February 1818 and had three children James that was born in July 1823, William that was born in November 1825 and Catherine that was born in January 1834.
From those facts I infer that Catherine, James and William of item 2 and 3 could be the same than those of item 4 and that John and Kate get married in Ireland before going to Argentina. The problem to go on with my research is that I have no access to the Irish parish records to get more info. If from the above mentioned facts anyone of you can infer more info, please let me know.
I have many docs of the Argentine branch of McDonnell-Duffy. If anyone of you want I will send you a copy.