The centerpiece of “Love, Weddings & Other Disasters” is a sweetly engaging story about the improbable autumnal romance between an impossibly demanding celebrity caterer, played by a perfectly cast Jeremy Irons, and the age-appropriate, sight-impaired free spirit (a very winning Diane Keaton) who manages to locate a warm heart beneath his frosty demeanor. Unfortunately — indeed, tragically — this appealing narrative is paid only sporadic attention during the run of writer-director Dennis Dugan’s haphazard amalgamation of loosely interconnected storylines that, save for the scenes involving Keaton and Irons, run the gamut from blandly predictable to overbearingly unfunny.