It comes from an article by Colin Coyle in the (Irish)Sunday Times which is unfortunately not online, so I'll just quote the relevant pieces:
Supporting Glasgow Rangers in Dublin must fell like living behind enemy lines.
But when 35-strong Dublin Rangers Supporters' Club raised a banner bearing that inscription at a recent match against Falkirk at Ibrox stadium, police ordered them to remove their "sectarian" banner.
The flag, which bears the legend Behind Enemy Lines, along with the Rangers crest and the Scottish and St. George's Cross flags, was confiscated at the match on the instructions of police, for fear of it inticing sectarian hatred.
The 20 travelling Irish supporters, who claim they have fallen foul of political correctness, were warned by stewards not to display the flag at subsequent matches. Steve Clark, co-founder of the supporters' club said the reference is a tongue-in-cheek allusion to supporting the club in a city dominated by fans of Glasgow Celtic, Rangers' greatest rivals.
"Its a cheeky bit of fun, nothing more. It has nothing to do with the troubles or bigotry," he said. "We flew the banner against hearts in December and previously at an away match in Barcelona and had no problems, so we were very disappointed by the police's stance.
Actually, slight correction, as you can see here, it's the Cross of St Patrick not that of St George on the banner:
You can read more about the Dublin Loyal and their campaign to have the banner reinstated at Ibrox on their website here(the photo originally came from there also).
A bit more from the article:
In a statement, Glasgow Rangers said it fully supported the police's decision to remove the banner. Rangers fans in Dublin already live a surreptitious existence meeting at a private members club to watch matches.
"We have normal working-class Dubliners in the club, with no links to Glasgow, but none of us would wear a Rangers jersey in a Dublin pub, as it's asking for trouble," said Clark. "If we're attending a Rangers-Celtic match, we fly out of Belfast, where wearing a Rangers jersey is more acceptable and less likely to lead to a confrontation."
And here was the actual reply sent by Rangers FC to their Dublin fans:
The club position on this banner and the position of many others I have spoken to is that this slogan is not humorous at all and reflects back to the period of trouble in Ireland and such references are not welcome in our club.
It is not reasonable to suggest that you could be arrested for cracking a joke. In fact it is patently ridiculous. No one has ever been arrested or banned for this at our stadium. People are arrested for breaking the law and have the right to dispute this in court.
In relation to the slogan in question can you tell me where the humour is? Also can you tell me who the enemy are? Finally where are the enemy lines referred to in this slogan. Many objective observers would ask the same questions.
I regret I can be of no further assistance to you on this subject.
I also don't think the slogan is that funny, but come on, Rangers need to get a grip here...compared to what else can be seen and heard at Old Firm games (and material sold outside the ground), it's pretty harmless, if a bit infantile.
Anyway, as is usually the case with this kind of episode, the whole thing has now a momentum of its own with a petition backing the "Dublin 20" with thousands of supporters signing it. And The Dublin Loyal's St Patrick's Night Bash ("Celebrating what's best in Ireland") will no doubt now be a sell-out.