Interesting - why do you think that this is an identity theft attempt?
I would be concerned if someone asked me for information on a living person. At least in this case, the person he is asking you about died in 1879.
Also - he is actually offering information, rather than asking you about sensitive information.
It seems like you could be OK if you keep your discussion to his Leddy family connection to Patrick Collins. Unless I'm missing something, there doesn't seem to be much of an identity theft risk if you exchange information about this very specific topic.