Like many people here, I started out finding my patience being tried by this film. By the end, I actually shed a few tears.
It seems to be in the nature of most old films to drag for 7/8th length and then catch fire right at the end. Older film-goers learned to bide their time patiently through the slow parts, calm in the knowledge that the big payoff is on the way. But that isn't quite accurate. You see, to earlier audiences, what are to us the "slow parts" were the main body of the story. They watched and found anecdotal and thematic interest there. Modern audiences, post-Spielberg, are in a constant state of waiting to be hit with a small climax every two minutes when they see older films. It's the inflation problem of modern movies. Well, that isn't going to happen. It is not necessary to apologize for these films; it is simply that you have to adjust your expectations and personal rhythm when you watch them. At this point, the difference between Avatar and The Informer is like the difference between Euripides and a traveling production of Rent. Think about it for a minute or two. Not to strain at the obvious, but Euripides still deserves a hearing.
The "exciting part", for most modern viewers, begins with the IRA tribunal scene and escalates to the final couple of minutes, which, if you are at all on board or even paying attention by that time, will tear your heart out. It's not some high-tone universal abstract plea for forgiveness; it's a plea from one dimwit, and those who feel sorry for the big lummox, for a little mercy. It's that personal, and that embarrassingly naked an appeal. For after being mad at Gypo, irritated at him, thinking this is the dumbest character of all time, you finally find yourself won over by the scene of Gypo's erstwhile girlfriend pleading to another woman to talk her man into going easy him.
The film may be sentimental, but the sentimentality is not cheap as some here have charged. There's a matter of life and death that plays out here, and as long as you take the proposition of one life to a customer seriously, it's sentimentality wrung out of the most serious stuff.
8 of 10. And the fault for it not being 10 of 10 is my own and in some measure yours, if you are reading this. We have all asked for more, ever more, faster, ever faster until we cannot put ourselves in 1935 -- just yesterday, really -- as easily as we should be able.