Both Achilles and Cu Chulainn were sons of a god and a royal human. Thetis (goddess) and Prince Peleus (mortal) were Achilles' parents. Cu Chulainn's mother was Deichtire, sister to King Conchubar. Some say his father was Lug, the Celtic god of light, while others say his father was an Irish chieftain.
Both had special horses. As a wedding gift, Peleus and Thetis received two immortal horses named Balius and Xanthus. They came to be Achilles' horses. Xanthus spoke to Achilles before his death.
Cu Chulainn was found as a baby by the men of Ulster, and as they were taking him home, they heard a horse whinnying nearby. They stopped to check it out and found a mare who had just dropped her foal. They took the newborn horse, saying he was meant for the special baby. The Gray of Macha, as the horse came to be called, served the warrior well in battle and was cognizant on an almost human level. Born the same day, they died the same day.
And now, moving on to the most important part. The greatest similarity between the warriors is their mindset and their fates.
In History 261, I learned that Achilles had a choice. He could become the greatest and most renowned warrior ever and live a short life, or live a peaceful, normal, and long life. He chose renown and died in the Trojan War. The funny things is, Cu Chulainn had the same choice.
When Cu Chulainn was just seven years old, he heard Cathbad the Druid tell his pupils that whoever took up arms that day would become the greatest warrior ever but lead a short life. Immediately he took up arms.
Cu Chulainn explained, "For when they asked him what special virtue lay in this day, he told them that the name of whatsoever youth should therein for the first time take arms, would top the fame of all Erin's men; nor thereby should he suffer resulting disadvantage, save that his life must be fleeting short."
Cathbad confirmed his words, "noble and famous indeed thou shalt be, but transitory, soon gone."
"Little care I," said Cu chulainn, "nor though I were but one day or one night in being, so long as after me the history of myself and doings may endure." The quotes are from The Boyhood Deeds of Cu Chulainn.
And so, the great Greek warrior Achilles and the great Celtic warrior Cu Chulainn faced the same decision, made the same choice, and died young but are still remembered today.
The Cu Chulainn stories (except the story of Cu Chulainn's birth) are from David Greene's An Anthology of Irish Literature, vol. 1
James Hunter's Achilles article
Thetis and Peleus
The picture of Cu Chulainn was cropped by Edana A. Click on the caption to see the original.