Sometimes siblings communicate with each other in a secret, private language that is unintelligible to the rest of the world. In Conor McPherson’s play, “The Seafarer,” the language is Irish.

Not Gaelic Irish. It’s English-Irish, actually, spoken with a brogue so thick and obscure that it might as well be Chinese.

The title of the play is taken from an old English elegy, a poem that is defined as “a sorrowful piece of writing.” Long the subject of scholarly controversy, the poem is most commonly viewed as an allegory, a psychological monologue by a “penitential exile” who sees all life on Earth as like death.

In McPherson’s play, the “penitential exile” is “Sharkey” Harkin (Andrew Connolly) who has returned to his childhood home in a suburb of Dublin to tend to the needs of his blind brother, Richard, played with vicious hostility by John Mahoney. Sharkey, in his long years of wandering, had apparently committed a crime. His self-imposed penitence seems to be to endure and obey his brother’s relentless demands.

It is Christmas Eve morning as the play opens and slowly — very very slowly — introduces us to the brothers and the dismal state of their relationship. There is a lot of talk about feces (Richard spends a long time in the bathroom, shouting through the door) and complaints about vandals who have apparently defecated in the back yard, leaving a mess that Richard orders Sharkey to clean up.

Richard, who has awakened with a hangover, begins drinking again immediately and doesn’t stop throughout the long day. Sharkey, an alcoholic, has given up liquor and manages to refrain from drinking even when their home is “invaded” by friends paying a Christmas visit. Paul Vincent O’Connor plays their likable, loyal friend Ivan and Matt Roth plays Nicky, a man who has earned the enmity of Sharkey and is an unwelcome guest. Moreover, Nicky has brought a friend with him, a suave, sophisticated gentleman named Mr. Lockhart (a smoothly sinister Tom Irwin).

The friends have come to play cards. And, as it is revealed, Mr. Lockhart, who turns out to be the devil, has played cards with Sharkey before — and lost. The game, therefore, is being played for more than ordinary stakes.

The only consistently intelligible monologues are delivered by Mr. Lockhart, who ruminates on death and heaven and hell with a melancholy brilliance that, in my view, is the essence of the play. Irwin, who has been a member of the renowned Steppenwolf Company since 1979, is well remembered here as the amnesiac lead in “Pyrenees,” which ran at the Kirk Douglas Theatre in 2005.

Playwright Conor McPherson directed his own play, “The Seafarer”, when it opened in London in September 2006 and on Broadway in December 2007. Jim Norton, who played Richard in both productions, won an Olivier Award and a Tony, and McPherson was nominated for an Olivier for Best Play and Tony awards for Best Director and Best Play.

In the current production at the Geffen Playhouse in Westwood, Randall Arney, the Geffen’s artistic director, serves as the play’s director, and while the staging is fine, and the dialogue sometimes even funny, Arney doesn’t seem able to control either the unintelligible delivery of his actors nor the persistently over-the-top shrillness of Mahoney. Which is unfortunate, since Mahoney was universally loved in his long-time role as Frasier’s dad and has shown himself, once again, to be an absolutely stunning actor in the current season of HBO’s “In Treatment.”

Probably the best thing about “The Seafarer” is the shabby, cluttered set designed by Takeshi Kata. Even more effective when contrasted with the richly elegant set this award-winning designer produced for the Geffen’s production of “Boston Marriage” in 2005.

Janice Pytel has designed appropriately suitable costumes and Daniel Ionazzi’s lighting design effectively illuminates the characters of the players, especially Sharkey, as he obsessively turns the lights on and off.

If one doesn’t know that “The Seafarer” title refers to an old English poem, he might come away with the nagging question, “What does the title have to do with the play?” But now that I’ve answered that question for you, you can go and enjoy the play (or not) at The Geffen Playhouse, 10886 LeConte Ave., in Westwood, Tuesdays, Wednesdays, and Thursdays at 7:30 p.m., Friday at 8 p.m., Saturdays at 4 and 8:30 p.m. and Sundays at 2 and 7 p.m. through May 24. Call (310) 208-5454 for reservations.

Cynthia Citron can be reached at ccitron@socal.rr.com.