My old man says the original "Huns" were Celtic when he was going to the football. I believe this changed late 70s when the Rangers away support were christened Rampaging Huns by the media and this was picked up and used by the opposition fans.
However, the term in Northern Ireland is a bit more sinister with KAH Kill All Huns graffiti shown in Nationalist schemes. This does not mean solely that Hun = Rangers Fans across the water - I believe it is a derogatory term for all Unionists / Protestants.
It's a term that doesn't particularly bother me but it's one best left alone in my opinion.