(written from a Production point of view)
Mickey J. Cassidy is a stuntman and stunt actor who portrayed the mirror universe character Private M. Romero in the Star Trek: Enterprise episode "In a Mirror, Darkly". He received no credit for his appearance. His costume was sold off on the It's A Wrap! sale and auction on eBay. 
Cassidy is the son of stuntman and stunt coordinator Michael Cassidy and the brother of stunt performers Zane Cassidy and Mariah Cassidy. Because of his fathers connection to the stunt business, Cassidy got at early age into the movie and television business. His first experiences were the science fiction fantasy film The Ewok Adventure (1984, with Fionnula Flanagan and Darryl Henriques, stunt coordinated by his father and with stunts by Frank James Sparks) and the family comedy Honey, I Shrunk the Kids (1989, with Matt Frewer, Amy O'Neill, Mark L. Taylor, Frank Welker, Craig Richard Nelson, and stunts by Cherie Tash). For the letter one he performed three high fall stunts and finally earned his SAG card. Cassidy doubled actor Macaulay Culkin on most of his '90s films and set a world record at the age of 13 while performing a 170' decelerator fall off a cliff on the drama The Good Son (1993).
Cassidy doubled lead actors, portrayed supporting roles, and performed stunts in feature films such as the comedy sequel Problem Child 2 (1991, with Zach Grenier, Diane and Elaine Klimaszewski, and stunts by Caron Colvett, Gene LeBell, and Zane Cassidy), Steven Spielberg's fantasy adventure Hook (1991), the sport film D2: The Mighty Ducks (1994, with stunts by Brian Avery, Christian J. Fletcher, and Pat Romano), the comedy Getting Even with Dad (1994, with Saul Rubinek and Ron Canada), the animated fantasy film The Pagemaster (1994, with Ed Begley, Jr., Christopher Lloyd, and the voices of Patrick Stewart, Whoopi Goldberg, Leonard Nimoy, Robert Picardo, and Frank Welker), Richie Rich (1994), Casper (1995), and the comedy Spy Hard (1996).
He has also performed stunts in David E. Kelley's acclaimed drama series Picket Fences (starring Ray Walston and Kelly Connell) and portrayed supporting parts in the drama The Adventures of Huck Finn (1993, with Leon Russom and Dion Anderson) and the comedy Free Enterprise (1998, starring William Shatner and with Thomas Hobson, Diana Cignoni, Deborah Van Valkenburgh, Chase Masterson, Mark A. Altman, and Daren Dochterman).
After a longer break he returned in 2003 he has doubled several leading actors in the daytime soap The Bold and the Beautiful (2003-2006), including Jack Wagner, Dax Griffin, and Matthew Borlenghi. Cassidy has also performed stunts in Sam Mendes' drama Jarhead (2005), the television special All That 10th Anniversary Reunion Special (2005, alongside his father and brother), and in an episode of Ned's Declassified School Survival Guide (2007, with Rob Steiner and stunts by Vince Deadrick, Jr., Mark Ginther, and Danny Downey).