There may be no greater drama in sports than a big heavyweight showdown. Seamus McDonagh, who fought Evander Holyfield in 1990, is quite familiar with that setting.

Now, he is about to familiarize himself with another form of drama. McDonagh plays the lead role in the one-act play, “Kid Shamrock,” which is opening Off Broadway later in June. McDonagh, the former New York Golden Gloves champion, has performed in several theater productions since retiring from the ring in 1991.

“Like boxing, acting is a solitary endeavor,” said McDonagh. “You are basically all alone on that stage and you better have a plan for living.”

The story of “Kid Shamrock,” was written by Robert Cassidy Jr., a former columnist for It is based on the life and career of his father, the one-time middleweight and light heavyweight contender, “Irish Bobby” Cassidy. The play depicts the rollercoaster career of “Kid Shamrock,” and one of its themes is the fighter’s struggle with alcohol, a topic in which both Cassidy Sr. and McDonagh can relate.

The production also stars Patrick Joseph Connolly, Shane McCaughey, Shannon Lower, David C. Morgan, the boxing author, Ron Ross, and Cassidy is expected to make a cameo during the evening. Connolly returns to the stage after making his feature film debut in “The Gardener of Eden,” at the TriBeCa Film Festival. He also has a role in the final episode of “The Sopranos.”

“I feel very fortunate to be part of this production,” said Connolly. “Above everything else, I am a fan of the sport of boxing and a huge fan of Bobby Cassidy. I love to act and I love boxing, so this is like a dream come true.”

Connolly, 53, grew up in Levittown, Long Island, and followed his older brothers to watch Cassidy fight at Sunnyside Garden in the 1960s and 1970s. Fast forward two decades and McDonagh was the hot Irish pug filling the fight joints in New York City. Speaking about the play, and his life since retiring, McDonagh put a new spin on his favorite old quote, the one Billy Conn used after losing to Joe Louis.

“If I didn’t try to knock him [Holyfield] out, I would have won easily,” said McDonagh. “But I would not have had this opportunity or the experiences I have had since that night to discover that there is more to life than seemingly winning. This play has had me relive boxing and finally get to enjoy it.”

McDonagh, 44 and now living in San Francisco, had a bout with the bottle but has been sober for 11 ½ years. Cassidy Sr. has not had a drink since 1974. The lessons they learned along the way serve them far better than knowing how to hook-off-the jab.

The evening is being directed by theater veteran Gary Morgan and includes a multimedia presentation by Chris Cassidy, a reading by Ross from his acclaimed book, “Bummy Davis vs. Murder Inc.” as well as a boxer’s roundtable hosted by Showtime analyst Steve Farhood. He’ll be joined on stage by Juan LaPorte, Carlos Ortiz and other fighters.

The event takes place at the Producers Club Theater at 358 W. 44th Street and opens on Friday June 29, with two additional performances on June 30. For tickets, call TheaterMania at 212-352-3101 or visit their web site,