With slightly more wit, better casting, and finer handling of tone, “My Brothers,” the directorial debut of Irish screenwriter Paul Fraser, could have been a perfectly pleasant, bittersweet account of a dysfunctional yet loving family, like so many adequate little films before it.

But in the hands of Fraser and writer William Collins, the quaint-little-film mentality of “My Brothers” is forced from start to finish. The story of three cloyingly dissimilar Irish siblings on a road trip to replace their dying father’s broken wristwatch, “My Brothers” wants so badly to be loved it forgets how grating its characters are.

For starters, the three leads never seem like brothers. Noel (Timmy Creed), the eldest, is supposed to be a senior in high school, but with his stubble and weary eyes, he appears just shy of 30. He’s supposed to be the “responsible” sibling, but he registers as a complete doormat, allowing his pudgy, bespectacled 11 year-old brother Paudie (Paul Courtney) to curse him out, win nearly every verbal sparring round and manipulate him into doing all the mechanical work on their vehicle. Meanwhile, the moppet Scwally (T.J. Griffin), an incorrigible space cadet, babbles non-sequiturs like a high-pitched Raymond Babbit.

The three are so disparate in temperament that you don’t want to see them bond; you want the ever-so-cute Scwally to be smacked and the relentlessly scatological-minded Paudie to be ditched on the side of the road. And when he does wind up isolated at one point, late in the film, Fraser goes for the jugular and puts him immediately in harm’s way of a pedophile, so you can’t enjoy his comeuppance. Bummer.

The film tries terribly hard to make us laugh; in scene after scene, Paudie and Scwally regale each other with the same fart jokes, fart noises, fart sing-songs. But how could Fraser and Collins not realize that the sound of two obnoxious punks shouting potty-mouthed nothings for five minutes straight is just plain irritating? They pick the cheapest way to characterize childlike vulgarity, and pass it off as endearing, as quirkily true to life.

Equally galling is the story’s rhythm-impaired gear shifting. Once the shit-and-piss humor has run its course, Fraser and Collins remind us this is a Serious Film About Brothers Growing Up, and so he assails us endlessly with the same lilting, melancholy classical guitar score, dictating exactly what to feel and when to feel it. Worse, Fraser apparently never noticed that the music drowns out much of the dialogue, which is frequently mumbled in an already impenetrable Irish brogue. (From this day forth, any Irish or Scottish film whose actors can’t master the peerless diction of Sir Sean Connery should include subtitles).

A few interchanges manage to ring true. It’s touching and funny, for instance, that Noel is ashamed of his father’s blue collar career as a bakery delivery truck driver, while the portly Paudie honors his dad for granting him so many free pastries. And the best scene involves Paudie’s slight maturation before our very eyes: the van breaks down opposite a girls’ soccer team practice, and having stolen Noel’s diary, he cheekily recites from it to the girls, expecting to find bad poetry and instead discovering a brutal betrayal. We see genuine hurt and insecurity come to his freckled face, and the sophomoric streak burns right out of him.

It’s an impressive acting feat from Courtney, a sort-of miniature wiseacre version of Drew Carey, and his character, at least temporarily, is given a second dimension. But no one else is allowed much range. Structurally, visually, “My Brothers” is mostly dreary, static, uninvolving, yet Fraser and Collins keep poking you to adore it. It has nice moments, but generally, it’s a film aching so hard to be raw and subtle, it winds up more oppressively sentimental than the schlockiest television soap operas.