First movie that pops in my head was “The Wind That Shakes The Barley” (2006) which has Cillian Murphy in it.

Here is what it is about:
Ireland 1920: workers from field and country unite to form volunteer guerrilla armies to face the ruthless “Black and Tan” squads that are being shipped from Britain to block Ireland’s bid for independence. Driven by a deep sense of duty and a love for his country, Damien abandons his burgeoning career as a doctor and joins his brother, Teddy, in a dangerous and violent fight for freedom. As the freedom fighters bold tactics bring the British to breaking point, both sides finally agree to a treaty to end the bloodshed. But, despite the apparent victory, civil war erupts and families, who fought side by side, find themselves pitted against one another as sworn enemies, putting their loyalties to the ultimate test.

Also there is a link that has the top rated Irish movies on imdb.com