It seems one of history's great ironies, but it's true. The crusader (well, it wasn't much of a crusade, but never mind that) Emperor, Frederick II, Stupor Mundi, was a direct descendant of the Prophet of Islam, Muhammad.
The pedigree is as follows:
Now, that's ironic.
Later addendum: After some further research, I can now reveal that many of Europe's nobility and royalty descend from Muhammad through Zaida (Isabella) of Denia. This applies to both King Juan Carlos I of Spain and Queen Margrethe II of Denmark. As a Dane, I find it particularly ironic that the titular head of the Church of Denmark descends from the Prophet of Islam.