The Brief:
IKR is an acronym for "I know, right."

DEEP DIVE
IKR is used as an expression of agreement. As the punctuation is nonexistent in the acronym-version of the phrase, context clues must be used to determine exactly what the intended tone is. IKR often connotes simultaneous shock and agreement.
Examples:
“Did you see what people are saying on Twitter about Prince William?!”
“IKR! It’s crazy! I’m disappointed but not surprised…”
ikr; shrek is great! can't believe you haven't seen it until now but it's whatever....
— Eddie (Edwardmck1/Edwardmck2) (@Edwardmck2) April 26, 2019
Ikr, it's like that sometimes
— Vote for BTS!!!! 💙👏🏽 (@ChimchimShamz) April 26, 2019