Currently iReal Pro doesn't support playing 8th notes on an off-beat.
Some industrious users have come up with work-around as described in a threads listed below:
https://forums.irealpro.com/threads/chord-changes-on-half-beats-up-beats.3260/
https://forums.irealpro.com/threads/how-to-synchopate-chords.5159/
https://forums.irealpro.com/threads/how-to-synchopate-chords.5159/