The Royal Irish Regiment is an infantry regiment of the British Army. Historically, it was known as the Royal Irish Rangers until 1992 when it became The Royal Irish Regiment. The regiment has a distinctive headdress that includes a green feather plume.

The green feather plume is a significant part of the regiment's ceremonial dress. It is worn on the right side of the headdress, which is typically a caubeen (a type of traditional Irish beret). The plume is made from emu feathers and is dyed green.

The use of the green feather plume is symbolic and reflects the regiment's Irish heritage and connection to the Emerald Isle. It is a tradition that pays homage to the historical significance of Ireland within the British Army and is a distinctive feature that sets the Royal Irish Regiment apart from other regiments.

Please note that the information provided here is based on knowledge up until September 2021. If there have been any updates or changes after this date, I would not be aware of them.