Installing mirror on the end of the handlebars

My Gepida Trekking says the handlebar is a Promax HB-3188. This appears to be made for Gepida. I have a small mirror I'd like to install in the end of the handle bar. Near the end of the handlebar there is a ring with two small hex screws. There's a plastic plug in the end of the bars. I loosened the ring, but it has nothing to do as far as I can see with removing the plug inserted into the end. I hate to pry out the plastic plug and risk damaging it if I decide the mirror is too small. Is there a trick to this? Are the plastic plugs a dime a dozen at any bike shop?