Inis
Can I play a Triskel in a clash I am not involved in?
Yes, unless the card text needs you to be involved. The designer says Battle Frenzy, Lug's Spear and Nuada's Sword only say "during a clash", so any player may play them. Ogma's Eloquence (played as a maneuver) and Diarmuid and Grainne ("one of your clans") do need you involved. Lug's Spear applies only to its own clash.