In coupling the Irish War of Independence and the Irish Civil War, Loach shows how a social revolution was sacrificed to a nationalist revolution that indeed led to a change in flag but not a change in fundamental structures of power. The film poses the stakes and the costs of both sticking to your principles and the practice of realpolitik.
We are living the consequences of both of these approaches today in American politics. Ideas that were once on the fringes of the right are now firmly ensconced in mainstream politics, precisely because there was a group that stubbornly persisted is promoting their once ridiculed agenda. At the same time, the left "compromised" away key principles in the name of short-term election gains. (I'm thinking here in particular of discourses of welfare reform and abortion, but it's happened in many other issues.)
What's more important? Concrete short-term change, or sticking to your guns however long it takes? Gaining access to power, or transforming the structures of power? Taking action only if it makes an impact, or dissenting no matter who sees it? What is the standard of effectiveness? These are all questions raised by the movie, and ones that I grapple with everyday.