Yes, Jon. Casement did "side with Germany," but you've got the wrong war: WW1, not WW2. It's not as if he were a Nazi collaborator.
But further, Casement's involvement with the Germans was a function of his Irish Nationalism; he had been involved in the Gaelic League and the IRB long before all that happened. Like many Nationalists of his day, he saw "England's difficulty as Ireland's opportunity"; seeking assistance from England's enemies when she is at war was a centuries-old tradition in both Ireland and Scotland. Casement was "pro-German" in the same sense that many other leaders and participants of 1916 were: Pearse, Connolly, Collins, de Valera, etc. Are they to be as vilified as Casement, too? Or just Casement, because he was the one who actually travelled to Berlin to ask for arms and troops?
Having now read up a bit on Casement, I also have to wonder if the special vilification of him at the time of his execution was out of what we today would term "gay-bashing." How many people who today see him as a traitor to Britain (or, on the other side, aren't too enthusiastic about including him in the pantheon of the martyrs of 1916), realize that those old attitudes were, at the time, closely intertwined with an anti-homosexual smear campaign against him between his arrest and execution?