I think it's probable that many people only know his first name as it appeared for many years on album covers and concert posters (and still does to this day, with his approval) as Paul. You're right, pdq, his friends do call him Noel, and so do many fans in private conversation, and it's how he introduces himself when you meet him; but onstage and throughout his professional life the group is still called Peter, Paul & Mary, and he still warmly answers to Paul.