Currently, user has to set a negative speed if he wants to animate its node from end to start. This patch modifies IAnimatedMeshSceneNode::setFrameLoop(start,end) so that the animation really starts from start even to end. This way animation speed is always positive.
http://irrlicht.sourceforge.net/forum/viewtopic.php?f=9&t=45908&p=264756#p264756
Patch
I'm having problems applying your patch, probably due to the fact that the file index is missing, along with revision metadata.