I would use the following to get the directory for your plug-in
import Rhino
import os
def GetPluginDirectory(plugin_name):
path = Rhino.PlugIns.PlugIn.PathFromName(plugin_name)
if not path: return None
return os.path.dirname(path)
if __name__=="__main__":
dirname = GetPluginDirectory("IronPython")
print dirname