Q: Cannot get into iPhoto library

You can open your iPhoto Library if you have the latest version of that app. If not, see the user tip, Get iPhoto 9.6.1 if you didn't update before OS X 10.10.3 by Barney-15E to learn how to get it.

However, if you are doing this in hopes of saving disk space, you probably will be disappointed. See Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support for more about that or Six Colors: The (hard) link between Photos and iPhoto for a more in depth look at how it is done.