Here is how I play Iris on standard tuning on my acoustic guitar. It sounds as good as the original and doesnt require all that nonsense of de-tuning strings.

Intro chords:(and verses)
E  0  0  0
B  3  3  3
G  2  0  0
D  0  2  0
A  0  0  2
E  0  0  3
(I just keep my ring finger on the "B" string (3rd fret) and use my index finger on the G and move it up a string for each chord. It's very easy. You'll get the hang of it if you just play with it)

Chorus:
(mostly) Bm  A  G
(then a small) G  F#m  G

For that little arpeggio before the pause I just play the "G" chord previously used (described below)
E  0
B  3
G  0
D  0
A  2
E  3
(but pick the notes in between strums; you'll have to listen to the recording to figure what I mean here. You'll have to practice a little to use your index and middle fingers while keeping the ring finger on the B string as mentioned at the beginning)
(Pluck notes on A & E strings sorta mid-strum holding the "G")
E  0  0  0  0  0  0
B  3  3  3  3  3  3
G  0  0  0  0  0  0
D  0  0  0  0  0  0
A (2) 0  0 (2)(0) 2