I am German and I have never seen the name spelled like this before. Every Dirk I ever met or heard of was spelled without an "e". So I don't know why this is given as the German version, but maybe it is old-fashioned. [noted -ed]
I'm afraid I have to contradict my fellow commentator here: I'm German, my brother is called Dierk with "ie", and I know at least one other "Dierk". It IS in fact the original German spelling (cf. German names like "Dieter, Dietmar, Dietrich" - a long "e" sound is usually spelled "ie" in German). The "Dirk" variant is originally Dutch, but has by now indeed become far more popular in Germany.