Shirei's ability only triggers if the creature's power immediately before leaving the battlefield is 1 or less, regardless of its power in your graveyard.

Shirei's ability triggers if a creature you own with power 1 or less dies under another player's control.

If Shirei leaves the battlefield and returns to the battlefield after a creature with power 1 or less is put into your graveyard, Shirei is considered a new object. The delayed triggered ability won't return the creature to the battlefield.

Shirei's ability can't return any creatures that died before Shirei entered the battlefield.