The term foreign fighters (FF) appears in numerous threads, there are two threads specifically on the theme and I understand during the peak of operations in Iraq (OIF) it was frequently raised - although I was unable to identify a specific thread.

Bob's World recently resurrected the issue on an old thread about the media in OIF and led me to think again about the issue.

The use of the term 'Wild Geese' IIRC comes from Irish history and I think is appropriate:
More broadly, the term "Wild Geese" is used in Irish history to refer to Irish soldiers who left to serve in continental European armies in the 16th, 17th and 18th centuries.
Link:http://en.wikipedia.org/wiki/Flight_of_the_Wild_Geese