Red deer have lived in Ireland for thousands of years. The most famous wild herd lives in Killarney National Park in the south-west, on the lower slopes of mountains and through ancient woodland. They have been there since before any of Ireland's cities were built.
A stag's antlers are made of bone. Strangely, they fall off every winter and grow back every summer, bigger than the year before. While they are growing, the antlers are covered in soft, hairy skin called 'velvet'. A full set can grow at over a centimetre a day.
In autumn, the stags 'roar'. It is one of the strangest sounds in nature - a deep, booming bellow that carries for kilometres across the hills. They roar to tell other stags how big and strong they are, hoping the others will walk away rather than fight.
Deer are mostly quiet, gentle animals. Most of the year they graze peacefully in small family groups. They are also extremely good at hiding - a red deer can stand completely still in a wood and be only a few metres from a walker, who never notices it.