The movie is Widow's Peak, a gentle Irish comedy set in the 1920s. It is being filmed around this tiny lakeside village (population 172) in the rolling green hills of County Wicklow.
But it's the other not-so-gentle cameras that Farrow fears. The paparazzi are out in force - lurking close to the grimy windows of the pub, hiding in cramped doorways along the single main street with its scattering of small houses, or skulking behind trees.
The situation has been made more difficult by a subplot outside the script: the unheralded arrival last month of Allen, insisting that he see his son by Farrow - Satchel, age 5. Farrow refused. Allen obtained a court order in New York. Farrow reluctantly relented.
Allen took his son for a brief outing to the Dublin zoo, where they looked at camels, monkeys and a baby giraffe. The paparazzi cameras went into ecstatic overdrive. The boy was driven back to his mother after three hours, and Allen flew on to London to publicize his new film, Manhattan Murder Mystery.
This is the kind of distraction that a film company can do without in the middle of shooting a movie, especially one as intricate as Widow's Peak.
The ripples of the brief encounter are still clouding the water, and the tension is palpable as Farrow tries to forget the emotional blip and return to the script. Allen is out of sight, but not out of mind.
Farrow appears suddenly out of a side door. Blond hair untidy around her shoulders, her slim figure clad in a blue silk '20s-style dress and brown lace-up ankle boots, she hurries through the puddles to the safety of her trailer. The door shuts on the outside world. Three stolid blue-clad gardia (police) move out to block unwanted intruders.
Inside, Farrow curls up on a brown settee, shakes her mop of fair hair and declares: "Making this movie is like therapy after these past horrendous months I've lived through. Do you know it's the first film I've done without Woody Allen in 13 years? No wonder it feels so strange to find a new face behind the cameras!"
The film she has chosen for "the start of my new life" centers on a group of women in a small Irish town run by a despotic matriarch (Joan Plowright). They are all widows, apart from shy and demure Miss O'Hare (Farrow) who is being courted by the local dentist (Jim Broadbent). Miss O'Hare is shy, that is, until the arrival of a beautiful war widow (Natasha Richardson) changes the mouse into a tigress, all claws bared, and causes havoc in the tight-knit community.
The "new face" behind the cameras on the $7 million revenge comedy belongs to British director John Irvin, a reassuring presence who has an impressive track record, including movies such as Dogs of War and Hamburger Hill and the classic John le Carre TV spy serial, Tinker, Tailor, Soldier, Spy.
The burly, 6-foot veteran acknowledges: "I am working with an artist who has been through a lot of pain, and she may still be in a lot of pain. I do what I can to help. I would be a lousy director if I was insensitive to anybody's suffering." The pain still shows. Up close, Farrow seems vulnerable, edgy, fragile. She could pass for 30, although, astonishingly, she is 48 and was married to two show-business icons - Frank Sinatra and Andre Previn - before her long relationship with Woody Allen.
Farrow reaches for the first of the 20 cigarettes that she will smoke this day. A light laugh. "During the case, I found I needed a vice, real quick," she says. "I didn't want to overeat or hit the bottle - so I took to smoking! Now I start with three a day, but on a good night, hanging out with the crew in the pub, I might get through a whole pack."
The "case" was a gripping 10-month drama of anguish, recrimination and naked hatred played out in front of the media as Farrow fought for and won custody of the children she shared with Allen - Moses, 15, and Dylan, 7, who were adopted, and Satchel - all three of whom were with her in Ireland. "I came perilously close to a genuine meltdown of my very core," Farrow says.
A hushed courtroom heard how Allen, the man who had been Farrow's partner for 13 years, had been having an affair with her adopted daughter, Soon-Yi Farrow Previn. Farrow, in turn, was accused of "twisted and bizarre" behavior.
"I regret the day I ever met him," she says of the man who transformed her career with films such as The Purple Rose of Cairo and Hannah and Her Sisters. "I hope I never see him again."
The public mudslinging left its stains. But now Mia Farrow tilts her chin and says: "I feel like I'm out of a cage. I mean, Woody's films were all so - insular. He had his own family of actors, and everyone knew his movies took up most of a year. So I never got the chance to do anything else.
"Now the scripts have started coming through again, and, please God, may they continue. I've got a lot of kids (seven in addition to Satchel, Dylan, Moses and Soon-Yi), and I need the money!
"I couldn't wish for a better film to start my career again than this one. Or a better director. I love John! I find him incisive, positive - he's not a great talker, he doesn't go on and on about what you should do, the way Woody did, soul-searching and analyzing.
"John is a man of few words - but those few words are magic, just what I need to hear. You have to trust your director, and I trust John absolutely, more and more every day. It's a treat to work with him. It's been a delight, everyone has been so supportive. And coming back to Ireland - I feel I've come home."
This home is the picturesque hamlet of Ballyknockan, a few miles from Dublin and seemingly a million miles from New York. And the other cast members are fully aware of their supportive roles. Natasha Richardson, 29, daughter of Vanessa Redgrave and director Tony Richardson, plays Farrow's rival in love. She says: "We all know what Mia has been through, though we don't mention it. I do feel a strong sense of protectiveness toward her.
"We'll always be supportive. Film companies become families, some closer than others. And on this one, we're very close. When the paparazzi come and try to sneak photographs of her, we all rally round just like any family would do if one of them needed us."
Plowright, the 64-year-old widow of Laurence Olivier and a grande dame of the theater, confirms it and adds, "She's getting a lot of support - but Mia is Mia, with her own inner quality that is always there on film, and it's there now. She's doing just fine."
Indeed, when Allen walked in, one wag joked: "That's putting the pigeon among the cats!"
Ireland, Farrow's friends believe, will do much to heal the wounds. The Emerald Isle has never looked greener, mainly because of the rain that has fallen over the Wicklow Mountains, where the thunderclouds hover like bursts of muddy gray surf above the slopes.
Farrow has strong Irish connections - her actress mother, Maureen O'Sullivan, grew up in a large Catholic family in a mansion overlooking Killiney Bay. And by a remarkable coincidence, playwright Hugh Leonard wrote Widow's Peak for her mother 10 years go, with the idea of Farrow playing a lesser role.
The playwright, now in his 70s, could hardly have imagined that one day it would be nicknamed "Woody's Pique" - but that's show business.
"It's a marvelous script," Farrow says. "Out of all the ones that are coming my way, it was the most seductive." She flicks off the choices, a finger pressed to her lips. "I did wonder what it would be like to act again - without Woody. I've gone through so much. But I know I've made the right choice.
"It's difficult to pin down what kind of woman (my character) is, because she's very complicated, not a simple character at all. That's what fascinated me about her.
"She's eccentric. She looks fragile. She seems to be a mouse - but she's not. She's got steel in her bones, oh yes, definitely! That's why I like her - I can identify with her. There's a lot of her in me, and myself in her - as I found out these past months.
"I've changed. You become very strong when your children are threatened. I know I did. I dug deep and found hidden reserves in myself that I never knew existed. Call the source of that strength religion. Call it God. All I know is that when I had to find it, when the crunch came, the strength was there."
When asked if there is a new man in her life, her smile becomes a snort of derision. Did someone really suggest that? "Any man would have to be mad to take me on - with all these children," she says. "And the last thing I need in my life right now is a madman!"
Mia Farrow throws back her blond head and laughs genuinely and unaffectedly: "My kids, my books, my music, my friends - that's all I want. But, please God, it'll be enough."