Daniel Cassidy, in his crazy book How The Irish Invented Slang, claimed that the word goon, meaning an idiot (and later, a muscle-bound henchman) derives from the Irish word guan, meaning ‘a fool’. There are several problems with this. Firstly, Cassidy states that the English word is ‘origin unknown’, while most dictionaries (including the OED) regard it as a contraction of an earlier word goonie or gooney, which is known since the 16th century and means a fool or a large bird like an albatross. This seems perfectly reasonable and I can see no reason to prefer an Irish derivation to this English origin.
Secondly, guan is not a common word in Irish. It is not given at all in Ó Dónaill’s dictionary, and in Dinneen’s dictionary it is ascribed to Ó Neachtain’s manuscript dictionary of 1730. The word guanach for silly or fanciful is certainly common and is given in all dictionaries but guan itself isn’t.
In short, this is not a completely stupid suggestion (unlike almost every other suggestion in this book) but given the existence of a strong English candidate, it does seem highly unlikely that Cassidy was right about this.