Born Thomas Cruise Mapother IV, Tom Cruise single-handedly tracked his family tree back to his great-grandfather, Thomas Cruise Mapother I. Great sleuthing, Tom!
A couple years ago he was reportedly "staggered" when handed a pedigree chart on national TV which had his immigrant ancestor, Welshman Dylan Henry Mapother, arriving in 1850. If he had been able to do any research on his own further than his namesake ancestor, his being staggered would have been justified: records from Castle Garden show his immigrant ancestor was born in Ireland, not Wales.
Although news to Tom, his Irish ancestry is not news to those who keep track of these kind of things: