You can use the OriginPath property to get the location of a plugin, but OriginPath is available in the scope of a plugin callback only.

__sifile__ and __sipath__, however, can be used in the global scope.

import win32com.client
from win32com.client import constants

import sipyutils

Application.LogMessage( "Global: __sipath__=%s" % __sipath__ )
Application.LogMessage( "Global: __sifile__=%s" % __sifile__ )

null = None
false = 0
true = 1

def XSILoadPlugin( in_reg ):
	in_reg.Author = "SOLIDANGLE"
	in_reg.Name = "TestPlugin"
	in_reg.Major = 1
	in_reg.Minor = 0

	#Register plugin items

	Application.LogMessage( "XSILoadPlugin: __sipath__=%s" % __sipath__ )
	Application.LogMessage( "XSILoadPlugin: __sifile__=%s" % __sifile__ )
	Application.LogMessage( "XSILoadPlugin: in_reg.OriginPath=%s" % in_reg.OriginPath )

	return true

The above will output something like the following:

# INFO : Global: __sipath__=C:\Users\SOLIDANGLE\Autodesk\Softimage_2015\Application\Plugins
# INFO : Global: __sifile__=C:\Users\SOLIDANGLE\Autodesk\Softimage_2015\Application\Plugins\TestPlugin.py
# INFO : XSILoadPlugin: __sipath__=C:\Users\SOLIDANGLE\Autodesk\Softimage_2015\Application\Plugins
# INFO : XSILoadPlugin: __sifile__=C:\Users\SOLIDANGLE\Autodesk\Softimage_2015\Application\Plugins\TestPlugin.py
# INFO : XSILoadPlugin: in_reg.OriginPath=C:\Users\SOLIDANGLE\Autodesk\Softimage_2015\Application\Plugins\