It’s the Lúin of Celtchar, which I’m intentionally conflating with the Spear of Lugh (no one needs two magical spears in one story, and anyway, some scholars argue it’s supposed to be the same spear anyway). That would make it one of the Four Treasures: https://en.wikipedia.org/wiki/Four_Treasures_of_the_Tuatha_D%C3%A9_Danann

    I love “research.” LOL