Well, from the foreground, it looks like your UV mapping may be "square", but with your texture being rectangular, it looks somewhat stretched. It's hard to tell if that's really what I'm seeing. If it is, you might want to correct the UV mapping, or just make the texture square.
Also, 2048x1024 is a pretty giant texture. I'd say it's way too much for sand. I'd shrink the texture and tile it more. If I wanted to get really creative, I might try adding a "detail map", though that would probably require some decent Irrlicht hacking (hmm... maybe I should look into doing that).
As for it not matching... I think that's just an effect of the mipmapping since the sand is fairly high frequency and high frequency gets lost with simple scaling algorithms.
Which renderer are you looking at it with? In my experience (other people seem to have the opposite), the OpenGL renderer does a much better job of keeping the high frequency data. I attribute this to the DX renderer using the software mipmap generation, which might benefit from having a slight sharpen added.
