He went to Germany to convince Germany to supply guns and ammunition for the rising. They agreed but the ship(Aud) carrying the supplies was captured. He was later caught and executed by the British for his involvement.
Roger Casement was an extremely brave man as he stood up to the wrath and barbarity of the British Empire in the Belgian-Congo he condoned the gruesome actions and left. He joined the Irish rebels and agreed to go to Germany in the search of arms, as John said correctly the name of the ship was the Aud. What is left out of many history books is that Casement didn't have a word of German and also he was a homosexual which was considered a crime at the time. You can imagine the Germans dealing with such a man. Robert Monteith who is largely forgotten by many actually managed to communicate with the Germans as at least he spoke German. They returned aboard the Aud with Captain Spindler but were reported off the coast of Kerry in Banna Strand. They arrived too early and someone reported them to local RIC barracks. They were subsequently arrested and Casement was hung in August 1916 in Pentonville prison. Ironically another 1916 leader Thomas Clarke had spent 15 years there before the rising. Hope this gives a better insight into the time. haha