If the targeted creature is an illegal target by the time Irresistible Prey resolves, the spell doesn’t resolve. You won’t draw a card.

If the affected creature attacks, the defending player must assign at least one blocker to it during the declare blockers step, unless that player controls no creatures that could block it.

You can target a creature you don’t control or that otherwise couldn’t attack that turn.

The creature affected by Irresistible Prey doesn’t have to attack that turn.