It was because there was a map of Monaghan on the banner and people were mistaking it for Iraq.
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Separated at birth? Iraq and Monaghan.
Association member John McKenna explained the problem to a news website: "It came as quite a surprise to us that Monaghan and Iraq had basically the same outline shape. We had been receiving some jeers and comments as we assembled for the parade in New York and we couldn't understand why. Until someone from the Louth Association pointed out the similarity. So for the sake of being able to walk 5th Avenue in peace, we had to carry a simple blue and white banner instead or our ornate traditional banner."