Trainers » Iris
Can you use Iris when your opponent has taken no prizes?
Yes, because it's possible for your opponent to take a prize during your turn if one of your Pokemon gets KO'd.
Source: TPCi Rules Team (2014-01-02), TPCi Rules Team (2014-05-08)
Trainers » Iris
Does the effect of Iris occur when you play it or when you attack?
Iris' effect looks at the game state at the time of the attack, not at the time when you play it.
Source: TPCi Rules Team (2014-01-02), TPCi Rules Team (2014-05-08)
Trainers » Iris
In a prerelease or booster draft tournament where each player starts with only 4 prize cards, will Iris add 20 damage even if no prizes have been taken?
No, it does not. Limited format tournaments may start out with fewer prizes than the standard six, but that does not mean those fewer prizes count as being taken.
Source: Plasma Blast FAQ (2013-08-15), TPCi Rules Team (2013-08-15)