(Above: the pilgrimage church in Wilparting, alleged home of the bones of Marin and Anian, although the monastery at Rott am Inn maintains that they have the real relics. Image found here.)
Well, Wikipedia will tell you that it comes from the Latin Ursus with the old German Perg, to mean “Bear Mountain” (there is a written reference to “Ursenperig” from 1315).
However, both Wikipedia and the book Unbekanntes Bayern (Unknown Bavaria), volume 4, refer to the legend of the Irish monks Marin and Anian who had settled there in the 7th century, bringing Christianity to the local inhabitants. Marin was martyred by marauding Vandals, and Anian, the story goes, died simultaneously of natural causes. They are referred to as Scotch-Irish, or Iroschotten. “Iroschotten” > “Irschen”.
Just something to think about next time you drive over the Irschenberg on the A8.
Added bonus trivia: the 2001 film Die Scheinheiligen takes place in Irschenberg, the plot revolving around construction plans for a fast-food reststop for the nearby Autobahn. (There is in fact a McDonald’s there now. The modern version of marauding Vandals.) The word scheinheilig refers to someone who is sanctimonious, hypocritical, holier-than-thou-but-faking-it.