Shoots the British official in this car: /vehicle_508233-Ford.html
I think this is a Royal Enfield Model 180 V twin like this one - http://buyvintage1.wordpress.com/page-18-1923-royal-enfield-ladies-model-225cc/ - but the bike in the movie has a much later era electric headlight fitted.
Brough Superior? http://en.wikipedia.org/wiki/Brough_Superior From the look of the colours it is most likely a WW1 era Triumph. Triumph Model H as built from 1915 to 1923
This scene was the reenactment of the assassination of Michael Collins; the Commander-In-Chief of the Irish National Forces who was shot by Denis "Sonny" O'Neill during an IRA ambush at Beal na Blath in the County of Cork on the evening of the 22nd August 1922. O'Neill was a former officer in the Royal Irish Constabulary who previously served in the British Army as a marksman during the First World War, he returned to Ireland in December 1918 and joined the ranks of the IRA. During the Irish War of Independence of 1920-1921, O'Neill was assigned as a training officer to the IRA Southern Command based at Mallow in County Cork where he met Collins serval times and later during the Irish Civil War of 1922-1923, O'Neill join the IRA Anti-Treaty Forces Cork Branch and was in the position when the Anti-Treaty Forces ambushed Collins' convoy. It was believed that O'Neill killed Collins on orders of Eamon De Valera as O'Neill was granted a military officer's pension by the Irish Government (under De Valera's rule) in the 1940s, while it's believed that Collin's death was a coincidence.