Few epics are as bloodthirsty as the Táin Bó Cúailnge, which tells of the war between two great kingdoms, Connacht and Ulster. It bears obvious resemblances to other national epics — the Iliad, say, or the Mahabharata. In this case, the war is over a prize bull, The Brown Bull of Cooley, and the warring factions are unevenly matched in the extreme, for the men of Ulster suffer from a terrible curse, imposed many years earlier by Macha, wife of Crunniuc mac Agnomain.

Macha, a legendary athlete, was forced to take part in a race against the King of Ulster's horses, even though she was nine months pregnant: she completed the race but collapsed at the finishing line and, in terrible agony, gave birth to twins. In revenge, she cries out that the Ulstermen will suffer the same agonies, for five days and nights, whenever they are in great difficulty or danger. Only women and boys and, fortuitously, the great warrior and magisterial tactician Cú Chullain are exempt from the curse.

Cú Chullain is one of the great creations of literature and it is in its portrayal of this great hero that The Táin stands out as a triumph of characterisation and psychological complexity. Cú Chullain is more complex than the usual hero figure and, with this valiant, wild, mercurial figure at its heart, The Táin becomes an epic masterwork, a recollection of great and bloody exploits, naturally, but also of psychological insight, and occasional flashes of dark humour, as in the passage where Cú Chullain meets the supposedly invincible Nad Crantail in single combat:

“‘We'll fire spears at each other,' said Nad Crantail, ‘but no dodging.'

‘No dodging except upwards,' said Cú Chullain.

Nad Crantail fired his spear at him but Cú Chullain jumped in the air as it reached him. It struck the standing stone and broke in two.

‘Foul! You dodged!' said Nad Crantail.

‘You're allowed to dodge upwards too,' said Cú Chullain.

Cú Chullain fired his spear, but upwards, so that it landed on the crown of Nad Crantail's head and went through him into the ground.

‘Good grief,' he said. ‘Truly, you are the best warrior in Ireland. I have twenty-four sons in the camp. Let me go and tell them about my hidden treasure. Then I'll come back and you can cut off my head, for if this spear is taken out I'll die anyway.'

‘Fair enough,' said Cú Chullain. ‘But do come back.'”

After this exchange, for all the world like two schoolboys squaring up in the playground, Nad Crantail tries to trick Cú Chullain, but is beheaded and quartered. The warrior Cú Chullain is both skilled and bloodthirsty and he can be terrible to behold, as in the massacre on Muirthemne Plain, where he kills “seven score and ten kings as well as innumerable dogs and horses, women and children, not to mention underlings and rabble”. Yet this is only one side of his character: in season, he is wise, thoughtful, lyrical, even a touch dandyish. The morning after the massacre, for example, he comes out to “display his elegant figure to matrons and maidens and young girls and poets”, and he takes extraordinary pains with his rather wonderful hair. Few epic characters are as psychologically complex: one moment tender, or funny, the next wading in blood and decimating whole armies.

That The Táin is a great read is beyond doubt — what is in question, perhaps, is the need for a new translation. The only easily available version before this was Thomas Kinsella's masterly rendering, with drawings by Louis le Brocquy, published in 1969 by Dolmen, and reissued by OUP. That, as Carson acknowledges in his generous and modest introduction, is a true classic, wonderfully vivid, perfectly judged, irreplaceable.

Yet Carson's version has its own virtues and rewards and is well worth acquiring — for the new reader, as a lively, engaging and wonderfully vernacular telling of a great tale, and for those familiar with the Kinsella, as an intriguing new perspective on a beloved classic.

The Tain translated by Ciaran Carson

Penguin, £15.99

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist