A beautiful Lament played by Davey Spillane on the uilleann pipes
From the Ireland/Éire Facebook page:
There's a story behind this lament, as there is behind any Irish lament (or Scottish one for that matter) but this one, I think deserves telling.
Now you have all heard of Cú Chulainn or Setanta, as he was called as a child. How he got the name Cú Chulainn or the "hound of Chulainn " is a different story, as is his battle to the death with his blood brother Ferdia. Now Ferdia was a very old and the closest friend of Cú Chulainn. The met in the land of Alba (Scotland) where was sent to train in the art of war and all the weapons, but that's again a different story.
This lament is about the time when Cú Chulainn killed his own son in battle you see you see when he was in Alba being trained by Scathach A fierce Scottish Warrior of renown he met Scathachs' sister. Both Scathach and Aoife, her sister were masters at warfare and had never been beaten, which is why the best young warriors were sent to train under her guidance. To cut this part of a long story short, Scathach and Aoife were feuding and after a long complicated story Cú Chulainn beat Aoife in battle and forced her to make peace with her sister, He also won her affection and so the inevitable happened and she had a son to him and she named him Conlaoch
Years later and not knowing he had a son, Cú Chulainn married Emer, his childhood sweetheart. When Aoife heard that Cúchulainn had married Emer, she was totally enraged. So she decided to turn her son into a weapon against Cú Chulainn. She trained her son in all aspects of being a warrior. She then sent him over to Ireland but first she put three geasa on him. A "geasa", for those who don't know is an unbreakable promise, your Word of honour, so to speak. Now for any true Irishman or Scottish man for that matter there is no worse fate than to be without honour. It is the single worst fate that can befall you. First of these geasa was that he was not to turn back, the second that he should never refuse a challenge, and the third that he should never tell anyone his name.
When Conlaoch arrived at his father's home in Dundalk, he was met by the warrior Conall, who according to custom asked him his name and lineage. Because of the geas his mother had put on him, Conlaoch could not comply with this request and was immediately challenged to a duel with Conall, which he could not refuse. After many such chalanges and battles, Conlaoch then came against Cúchulainn himself and was asked his lineage, but again could not tell it and so was challenged by Cúchulainn. In the terrible battle that followed the hero light came upon Cú Chulainn and Conlaoch realised that he was fighting his father and that his mother had been treacherous, he cast his spear sideways so that it would miss Cúchulainn and shouted that he was his son, but it was too late Cúchulainn had already thrown the gae bulga (which he had won from Aoife) and it was unstoppable once thrown and thus Conlaoch was slain.
Cúchulainn was thrown into a fit of rage and grief in which he lost his senses and started attacking anything in sight, so in order to save him and his friends from further tragedy, the Druid Cathbad cast a spell upon Cúchulainn causing him to see the waves of the sea as armed opponents. He battled with the waves until he collapsed from exhaustion.
So that being said this lament called "Caoineadh Cu Chulainn" (Cú Chulainns' lament) is about that tragedy.