I have recently started research on the Cairo branch of my family tree as well. I don't know if all the Cairos came from the same place in Italy, but I can tell you what I recently discovered.

I found my great-grandfather's name in the Ellis Island databank. He sailed from Naples to the Port of New York in 1908. He listed his birthplace as Rocca devandio, Caserta, Campania. He listed his ethnicity as So. Italian.

I have since learned that Campania is a region of Italy (of which Naples is the largest city) and Caserta is the province. I imagine that Rocca devandio was the town.

My next step is to see if there are still Cairos living there. Hope this helps. As I said, I don't know how common a name that Cairo is in Italy and whether they all came from the same region.