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:
And here was the actual reply sent by Rangers FC to their Dublin fans:
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.