To "fight like a Kilkenny cat" refers to an old story about two cats who fought to the death and ate each other up such that only their tails were left. There is also a limerick (with optional added couplet) about the two cats:
The story has many roots. One involves soldiers based in Kilkenny City. However, who exactly these soldiers were and when they were stationed in Kilkenny is subject to some conjecture. Some people think they were English and some think they were Germans under the pay of the King George III of England.
One version was told in detail in Notes & Queries in 1864. It was said that a group of German soldiers (Hessians) were stationed in Kilkenny, during the period of the 1798 rebellion. To relieve the boredom in barracks, sadistic soldiers would tie two cats together by their tails, hang them over a washing line to fight and place bets on the "winning" cat. Gambling was contrary to military regulations, the story goes that the soldiers, alarmed by the impending arrival of an officer, released the cats by cutting their tails with a sword. When the officer arrived and inquired about the scene facing him, he was told that two cats had fought each other so fiercely that all that remained were the two stubs of their tails on the ground.
A different soldier-based story tells that in the mid 17th century Oliver Cromwell's soldiers tied the tails of all the cats in Kilkenny in pairs of two and hung them over a wire. The cats then fought until they had killed each other. The final cat was then beheaded.
Another story has a thousand Kilkenny cats fighting a thousand cats from the rest of Ireland in a field outside Kilkenny City. All the cats died in battle. This may be a parable based on dissents of the period between the people of the Kilkenny area and other parts of Ireland.
After the Statutes of Kilkenny the city was divided into two townships called Irishtown and Englishtown, a situation that wasn’t uncommon in a country occupied for so long by the English. For religious, cultural and political reasons there were deep divisions between the two groups. This may lend itself to the story of two cats fighting. Because the rights and duties of the two townships hadn’t been made clear by statute this led to three centuries of dispute between the rival municipal bodies that ended in beggaring both of them.
The inhabitants of County Kilkenny are often referred to as Kilkenny Cats. The County Kilkenny hurling and Gaelic football team are well known as the cats and they wear black and amber colors, which are said to resemble the colours of a tom cat.