Information on the cameo
Michael "Mikey" Walsh is a character from the 1985 movie The Goonies. He is the leader of a young gang of kids called the "Goonies" and lives in the Goon Docks area of Astoria, Oregon. In the movie, he was portrayed by actor Sean Astin, who also portrayed Samwise Gamgee in Peter Jackson's The Lord of the Rings film series.
Appearance in the rap battle
Mikey Walsh, alongside Daenerys Targaryen, briefly appeared in the Great Hall during George R. R. Martin's first verse. He could be seen taking a breath of his inhaler while Martin said the line, "Ditch the Goonie and cast a couple boobies!"