I think she does actually have absent seizures and the members are probably aware, since during Tears, for example, when they're all jumping around and Irene is just standing still, they don't seem to take much notice, think much of it, or worry at all.
lost in thought maybe. or maybe when she's not in the center of attention she just kind of blanks out for a while, i know i do that. it also could be absence seizures like everyone in the comments says with how frequently it happens and how the members react to it (like it's normal for irene, they don't try to snap her out of it)
