Historically speaking especially in NYC, firefighters and police officers were Irish. When one of them died, he was given an Irish funeral. The Emerald Society, named after Ireland's Emerald Isle was created for such ceremonial functions. It has then become the standard regardless if the fallen is Irish/Irish background or not. If you notice with NYPD funerals, the casket is drape in stars and stripes but of green and white. I guess the Irish flag is a custom, symbolizing the roots and tradition of the ceremony .
