trigger MasterUserTrigger on User (
  before insert, after insert,
  before update, after update,
  before delete, after delete,
  after undelete) {

  if (Trigger.isBefore) {
    if (Trigger.isInsert) {


    }
    if (Trigger.isUpdate) {
    isrLeadSync ISRToSync = new isrLeadSync(Trigger.new);
        ISRToSync.syncISRs();
            }
    if (Trigger.isDelete) {

    }
  }

  if (Trigger.IsAfter) {
    if (Trigger.isInsert) {

    }
    if (Trigger.isUpdate) {

    }
    if (Trigger.isDelete) {

    }
    if (Trigger.isUndelete){

      }
  }
}

public class isrLeadSync {

    List<User> users;

    public isrLeadSync(List<User> triggerUser){
        users = triggerUser;
    }

    public void syncISRs(){
        for (User u : users){
        if(u.ISR_Owner__c != null && u.ISR_Owner__r.IsActive){
            List<Lead> leadList = [SELECT Id, ISR_Owner__c
                                     FROM Lead
                                     WHERE OwnerId = : u.Id
                                     AND ISR_Owner__c != : u.ISR_Owner__c];

        for(Lead l : leadList){
            l.ISR_Owner__c = u.ISR_Owner__c;
        }

        update leadList;
        }
        }
}
}