In my bathroom is a large glass mirror above the sink. I's say maybe 48x60 inches.
I would like to build a frame around it and I do not want to take it down. I know I'd need to get molding at the home improvement store, miter the corners, and paint pryer to hanging.

What I am not sure of is how to attach the strips to the glass and how to accommodate for those little plastic clips that secure the mirror to the wall. Anybody know?

I would measure the location of the clips, mark that on the back of the trim molding you are using, and notch the size of the clips out. To attach you may be able to find a clear adhesive if you are attaching the frame to the mirror itself. Otherwise, frame out around the mirror and attach to the wall. Hope this has been of some help.