Thanks for sharing with us, Original Raider! I'm certainly not the expert when it comes to Celtic/Saxon/Irish-(Catholic) historical and religious topics... to say I have but scarse hints and bits of info, so forgive me for my ignorance on that folklore element.
Conclusion: there are more than one source to this coin-laying ritual, and since the fim's partly about an Irish brotherband forged by common, deep ideologies for justice (or justified retribution), truth and equality, it's obvious your on a more precise track than me on this!
Again, thanks for sharing.