Further to my previous posts, The Centurion's Sarcastic Cry in Mark 15.39 and More on the Centurion's Sarcastic Cry in Mark 15.39, I would like to thank Earl Johnson for letting me know about two articles he has written on Mark 15.39, one of which is accessible free to all online:
Earl S. Johnson, Jr., "Mark 15,39 and the So-Called Confession of the Roman Centurion", Biblica 81 (2000): 406-413
E. S. Johnson, "Is Mark 15:39 the Key to Mark’s Christology?", JSNT 31 (1987): 3-22
I have not had a chance to read the latter yet, but the former makes a good case that we should not read this as a "confession" in the normally accepted sense of that word, though without specifically suggesting that the cry is sarcastic or ironic. See too comments by Neil Godfrey on Vridar, also with a mention of the articles by Johnson.