The breed is very old, possibly from the 1st century BC or earlier, bred as war dogs by the ancient Celts, who called them Cú Faoil. The Irish continued to breed them for this purpose, as well as to guard their homes and protect their stock. Regular references of Irish Wolfhounds being used in dog fights are found in many historical sagas - Cuchulain's favourite dog, Luath, was slain by a southern chief's hound, Phorp. While many modern texts state Irish Wolfhounds were used for coursing deer, contemporary pre-revival accounts such as Animated Nature (1796) by Oliver Goldsmith are explicit that the original animal was a very poor coursing dog. Their astonishing size, speed, and intelligence made them ideal hunting animals for both wild boar and wolves, and many were exported for this purpose. They were perhaps too ideal, as the boar and wolf are now extinct in Ireland. The Irish Wolfhound has been recorded as being exhibited in ancient Rome to some excitement, and mention is made that they so amazed and terrified the Romans that it was seen fit to only transport them in cages. There exist stories that in the arena, the original Wolfhound was the equal of a lion. It has also been shown that when hunting animals, the wolfhound would bite the neck and crush the spine, killing the creature. During times of conflict with England, it was not uncommon for Wolfhounds to be trained to take armoured knights off of their horses, thus allowing an infantry man to move in and finish the kill if the Wolfhound had not done so already. Due to a massive export into various countries as a gift for royalty and a ban that allowed only royalty to own such a dog, the breed almost vanished in the middle of the 19th century. Captain Graham rebred the Irish Wolfhound with the Deerhound, Great Dane, Borzoi and other breeds; this saved the breed, but had the inevitable effect of altering its appearance, most noticeably leaving the Irish Wolfhound with alternative colours such as brindle (inherited from the Great Dane) as before they were mainly grey in colour. The ancient breed (often referred to as the Irish Wolfdogge in contemporary accounts) was available in both a smooth and rough coated variety. Descriptions of its appearance and demeanor, as well as the method of its use place it closer to the flock guardians in appearance than the modern breed. It is clear that the dog was not always the giant of today and it has been suggested that the Wolfhound was part of the make up of the Kerry Blue Terrier. The historical variety was famed for its loyalty, discernment, grave nature and aggression. In terms of temperament the modern breed has been greatly mellowed. Wolfhounds are often referred to as "Gentle Giants", and an historic motto of the breed is "Gentle when stroked. Fierce when provoked." The Wolfhound is sometimes regarded as the national dog breed of Ireland but in fact no breed has ever been officially adopted as such. The Wolfhound was historically a dog that only nobles could own and was taken up by the British during their rule in Ireland. This made it unpopular as a national symbol and the Kerry Blue Terrier was adopted by early Irish Nationalists such as Michael Collins. Today, however, the Irish wolfhound is by some margin the best known Irish breed outside the dog world.

The Irish Wolfhound is a breed of dog (a sighthound) bred to hunt. The name originates from its purpose (wolf hunting) rather than from its appearance. These dogs are accepted as the tallest breed of dog in the world. The arguments as to whether the "largest" dog should refer to the height or the weight of the breed have their basis in these points of contention.[