In St. Luke's Gospel, there is mention of the Good Thief on the cross near Jesus who repents. No name is given him, but most say his name is "Dismas." Is this true?

We don’t know. The story itself is very moving and there is naturally a human tendency to want to know more. Thus traditions and legends often set up in cases like these. But the historical accuracy of such things is often difficult to assess. “St. Dismas” is a name that tradition supplies us in the Western Church. However other names have also accrued to him in the East and in other eras such as Titus, Zoatham, Demas, and Rach. 
 
Interesting though these traditions are, we sometimes miss the main point when biblical figures are not named. For, if you are prepared to accept it, you are the good thief who “steals heaven,” if you are willing to repent, take up your Cross, be crucified with Jesus and persevere to the end, asking God’s mercy and admission to his Kingdom. 
 
The “good thief” was not so much good as he was smart. He knew that he was a sinner, justly condemned, and that his only hope was grace and mercy. Having repented, he turns (which is what “conversion” means) to Jesus and in faith seeks his salvation. Jesus says, “No one who comes to me will I ever reject” (Jn 6:37). And thus he is saved. Smart!