As others have commented, the film does (on its face) have things to (potentially) enjoy: Judy's singing, Arthur Freed's handiwork, Cohan's songs, and a possible musical and comic look at Irish immigrants in New York. But in almost every way the film stiffs. Judy climbs from the wreckage (as usual) unscathed: her freshness and energy nearly making the film and songs rise to the level of entertainment. But George Murphy is the complacent stiff, the songs are mostly mediocre at best (except for a jazzy "Singin' In The Rain' by Judy), and worst of all "Grandpa" is a character who - although meant to be curmudgeonly and adorable in his irascibility - comes off as almost criminally abusive, ruining his daughter's one leap at married bliss, and doing a good number on his grand-daughter's as well. The actor in that role (the usually reliable Charles Winninger) gives an unlikeable and near one-note performance which constantly grates, until the viewer wants to strangle him. This may or may not have been purposeful, but - in terms of making the film (a light musical affair) bearable to watch - it is a disaster. You feel both aggravated by Grandpa's insistent hostility towards the happiness of others, and put off by the passivity of others toward his ugly and pointless behavior over the course of years. I don't think this is the stuff of light entertainment, but of a psychological essay. Grandpa's just a jerk...
It is true that every now and then you will find an unheralded film to be undeserving of its anonymity. This is not one of those cases: the film drags along, forgettable song after forgettable song, stiff actor after stiff actor, sentimental stereotype after sentimental stereotype, and all made worse by that horrible Irish stew pot of a Grandpa. This one can be skipped without feeling cheated.