Yes, go to EDIT ----> TRANSFORM ----> FLIP HORIZONTAL.
1. If you want to mirror ONLY A PORTION of the image, you have to make a SELECTION of that part then follow the above step.
2. If you want mirror the entire image, then just follow the above step.
STEP2 NOTE: I'm just basing of Photo 6.0, so if that image is a background image, you would have to duplicate the layer so that the TRANSFORM menu is actived.