Debra Messing, a nice Jewish girl, slides effortlessly into a foursome of otherwise Irish actors in the wistful “Outside Mullingar,” a new romantic comedy — that is, by the time it resolves an identity crisis — from award-winning writer John Patrick Shanley (“Doubt”).

A world premiere, “Mullingar” has just opened at the Samuel J. Friedman Theatre.

That Messing makes for a fine ensemble actress is hardly astounding. We saw it for eight seasons on TV's “Will & Grace,” in which she played a neurotic interior designer, and later in the Broadway-centric “Smash." What is surprising is how convincingly she plays an Irishwoman in her Broadway debut, which finds her cast as a thirty-something introvert on a farm in rural County Westmeath.

Maybe it’s the russet-colored hair? More likely, it’s that the Brooklyn-born actress plays Rosemary Muldoon as a strong and competent Irish archetype. Messing also knows better than to pull focus from her finely tuned co-stars, who include Brian F. O’Byrne (“Frozen,” “Doubt”) as an awkward misfit on the neighboring farm, and stage vets Dearbhla Molloy and Peter Maloney as, respectively, her aging mother and his ailing father.

Director Doug Hughes, who teamed with Shanley on “Doubt,” again takes the reins with “Mullingar,” which finds the playwright conjuring language as fertile as the fields we imagine separate the Muldoon and Reilly farms. An awkward land deal forged under dubious circumstances decades prior has left the Reillys needing to traverse 40 meters of Muldoon land to arrive at their own home, a fact that lowers the value of the Reilly home accordingly. (An illustrated Playbill map lays it out better than words could.)

As “Outside Mullingar” begins, the elderly Tony Reilly (Maloney) is threatening to disinherit shy Anthony (O’Byrne) to cede the property to a nephew in America. Rosemary, who has lived next door to Anthony all her life — pining for him, and holding a petty grudge — swoops in to remedy that situation.

It seems this land conundrum will be the through line of “Outside Mullingar,” but the plot element turns out to be a red herring. Soon, there’s a poignant scene between father and son in which past slights are forgiven and “Mullingar” morphs into a sentimental family reconciliation story. So, fine, anyway, dad shuffles off this mortal coil and … wait. Oh. Now O’Byrne’s out in a field, brandishing a metal detector — Rosemary mistakes it for a weed-whacker — searching for a trinket with some deep emotional relevance.

Is he going to stand outside Rosemary’s window hoisting a boom box, or whatever they’re using in rural Irish courtship rituals these days? No? Well, at least we’ve finally settled on matters. “Outside Mullingar” is a romantic comedy.

OK, so “Outside Mullingar” is actually all of those things, which left me a touch whipsawed. The erratic, or at least elastic nature of the script almost doesn’t matter, though, because the dialogue is so colorful (the college-aged victims of an auto wreck are said to be found with a “badger licking the blood” off their bodies — hey, this is an Irish play) and the acting so sharp, that the 95-minute, intermission-less rom-com, or whatever-this-is, seems to fly by.

Messing and O’Byrne give understated performances that exceed the sum of their parts. Maloney, who has appeared in “The Cripple of Inishmaan” and “Dinner at Eight,” may have been born to play a pale-faced, crotchety manipulator like Anthony’s dad. Molloy, Tony-nominated in 1991 for Brian Friel’s “Dancing at Lughnasa,” is stoic and occasionally cutting as Rosemary’s pragmatic mother.

Scenic designer John Lee Beatty joined Shanley and Hughes on a recent trip to the home of a Shanley cousin (named Anthony, no less) in the Irish Midlands. From that journey emerged Beatty’s notions of utilitarian kitchens and perpetually cloudy skies. Of note: Shanley’s Irish roots aside, this is the first of his plays set on the Emerald Isle.

Toward the end of “Outside Mullingar,” O’Byrne’s Anthony makes a revelation that explains his lifelong ambivalence toward Rosemary. It veers toward fabulism, and didn’t sit well for me, though I suspect if you’re there to see a love story, you won’t be disappointed, and may find it charming.

“Outside Mullingar” boasts actors at the top of their games and the dialogue you’d hope for from a Tony-, Pulitzer- and Oscar-winning scribe — theater PR types would like you to consider this an “Irish ‘Moonstruck,’” noting the screenplay that garnered Shanley an Academy Award. “Outside Mullingar” is unruly enough that you wish someone had been there to tame it a little more, but it’s still an easy play to adore.

“Outside Mullingar,” through March 16 at the Manhattan Theatre Club’s Samuel J. Friedman Theatre, 261 W. 47th Street. Tickets: $67-$127. Visit Telecharge.com or call 212-239-6200.

Follow Robert Kahn on Twitter@RobertKahn