Atsumori : 50 years in human world
Kōwakamai is a genre of medieval theatre (danse). Although it is less known, the text "jinkan gojūnen geten no uchi wo kurabureba" is very well known thanks to Oda Nobunaga. He was the most powerful daimyō (feudal lord) in second half of 16th century (at the end of Sengoku period) and died at age 47 because of the rebellion of Akechi Mitsuhide in June 21st 1582 in Julian calendar.
According to Shinchōkōki 信長公記 (Chronicle of Lord Nobunaga), Nobunaga danced Atsumori, one of Kōwakamai pieces (Kōwakamai is contemporary theatre for Nobunaga) before Battle of Okehazama against lord Imagaza Yoshimoto.
(See contemporary Japanese translation in this site: the column n°24 今川義元討死の事, about the death of Imagawa Yoshimoto in battle).
Pieces of Kōwakamai are so much inspired by battles and heros in the Genpei war (around 1180). There are some Kōwakamai pieces in kagura repertory as well. Sugisawa Hiyama which is one of bangaku (a style of kagura in Northwest Japan) is influenced by medieval theatres. I confirmed that a part of text of the piece Kagekiyo of Sugisawa Hiyama is same as Kōwakamai's Kagekiyo text (the part about Kagekiyo and his wife Akoya).
Taira no Atsumori 平敦盛 (Atsumori of Taira), nephew of Taira no Kiyomori 平清盛 was only 15 when he died in the battle of Ichinotani in 1184. He is young lord loving flute (hichiriki or transverse flûte, named aoba). This is given to his father from Emperor Toba.
The clan Genji (Minamoto no Yoshitsune) carrie out a surprie attack to clan Heike, and Heike was sclattered. Atsumori was late to take ship because he was looking for his flute. His enemy, lord Kumagai Naozane found him. Lord Kumagai stopped Atsumori who was going to escape saying that it is dishonorable to escape for commander. Lord Kumagai however didn't know that the worrior is a boy. Then Kumagai asked to Atsumori his name. Atsumori didn't answered and just let his head cut off.
Here is the most famous text of Atsumori in Kōwakamai text:
Jinkan 50nen geten no uchi wo kurabureba yume maboroshi no gotoku nari
50 years in human world is like dream and illusion in the world of geten (Buddhic world: Nirmānnarati in sanskrit).
Hitotabi sei wo uke messenu mono no arubekika
Once we are given life, no one doesn't perish.
Kore wo bodai no tane to omoisadamenuwa kuchioshikariki shidaizo
It is regrettable not to take this as a beginning of our awareness.
You have translation in article of Atsumori in nō theatre in wikipedia.
* Please remind that the wiki translation of 人間50年 is misreading. As Lord Nobunaga died almost 50 (I think he was 49 or 50 in traditional age counting), this verse is often misread like "the life of human being lasts 50 years, and it is like a dream in Buddhic world". Indeed, so many people remind this verse wrongly. In the linked videos, actors sing also "ningen" (correctly "jinkan'). Correctly, it may be read as jinkan. It is contrast between human world jinkan and fifth stage geten for the enchantement. This is a Buddhic word and nothing with ningen, the general term of human beings. (To confirm if the buddhic term first called as ningen).
** In the Buddhist thinking, the world is divided into six stages according to the level of awareness. One day and one night in geten corresponds 800 years in human world. People in this stage lives 8000 years. In this verse, it is said that comparing these enormous long terms, 50 years in human world is just like a short dream or illusion.
By the way, I have thought for long time that Lord Nobunaga danced Atsumori just before his suicide at Hon'nōji temple. But it may be my misunderstanding because of TV dramas.