Irresolute describes someone who feels stuck. A decision must be made, a plan acted on, but the irresolute person just doesn't know what to do.
Resolute describes certainty. When someone is resolute, things get done: plans are made and carried out. But add the prefix ir to resolute and you get its opposite. An irresolute person isn't necessarily a slacker — he or she just doesn't know what to do. Maybe it's confusion. Maybe it's a matter of waiting for better information to come along. Either way, if someone is irresolute, you'll need to be patient — or willing to nudge him or her into action.