I was originally thinking it was perhaps a town team but I could find only a few towns named Mullen. Mullen, NE has population of 491 as of 2001 so I could only imagine what the population would have been in 1915. In fact I can only find two cities named Mullen in the U.S., Mullen, NE and Mullen, CA.
http://en.wikipedia.org/wiki/List_of..._United_States
http://en.wikipedia.org/wiki/Mullen,_California
http://en.wikipedia.org/wiki/Mullen,_Nebraska