Dunno. Here's what some person or persons on Wikipedia had to say:
"The band was formed in 1946 shortly after the end of World War II by young Northern Irish war veterans in the townland of Drumalig, near Carryduff on the outskirts of Belfast. ... On settling on the name of the band the founding members wrote to Field Marshal Montgomery himself requesting permission to use his name. As well as answering in the affirmative Montgomery sent the band a donation of 10 shillings for band funds!"
Wow. I didn't know Monty was such a cheap bastard.