In Celtic mythology, the harp is often associated with the supernatural.
Irish folklore associates the harp with an ancient race of Fairy folk called the Tuatha De Dannan, meaning "People of the Goddess Danu." They were said to have been skilled in magic, and the earliest reference to them relates that, after they were banished from heaven because of their knowledge, they descended on Ireland in a cloud of mist. (Brittanica)
In Irish mythology, The Dagda was king of the Tuatha Dé Danann. According to Bard Mythologies “the Dagda had many wonderful possessions: his cauldron of plenty, which never ran empty, and could feed as many man as sat down to it; his mighty club, one end of which could kill a man, and the other end of which could restore him to life. But his greatest treasure was his harp, Uaithne. It was made of oak and encrusted with jewels, and only he could get music from it strings. The playing of this harp made the seasons come in the correct order, and could make anyone who heard it laugh for joy, or weep with sorrow.” (https://bardmythologies.com/the-dagdas-harp/)
These properties are know as the Three Noble Strains of harp music:
1) The Goiltai or ‘sorrow strain’ caused people to weep.
2) The Geantrai or ‘joy strain’ encouraged people to laugh.
3) The Suantrai or ‘sleep strain’ lulled people to sleep.
The Fomorians were great enemies of the Tuatha De Dannan. They believed that taking possession of this harp was the best way to defeat the Tuatha dé Danaan. The stole the harp, hung it on their banquet hall wall, and then feasted in celebration of their victory.
The Dagda was of course furious, and ventured out in search of his treasured harp. They eventually found the hall, and the Dagda burst through the doors. He summoned his harp which flew flew across the hall and into his hands. The Dagda then played The Three Noble Strains of music (The Irish Place).
First he played the Goltrai, the strain of sorrow, which made the Formorians cry uncontrollably. The tears filled their eyes til they could no longer see. In their despair, they reached for their weapons to try and attack.
Then, The Dagda played the next Noble Strain, Geantrai, the strain of merriment. This made the Formorians laugh. They laughed so hard that they dropped their weapons.
Finally, The Dagda played the third Noble Strain, Suantrai, the strain of sleep. This gentle music lulled the enemy into a deep sleep, leaving The Dagda and his warriors free to leave, taking the magical harp with them.
For more about the story see: https://www.theirishplace.com/heritage/irish-myths-and-legends/dagdas-harp-and-the-tuatha-de-danaan/