On the right side (we drive on the left in the UK) it is a good idea to fit a mirror.
It isn't the law, but I think it is a great idea for safety sake.
I couldn't go for a bar-end mirror, so got this clamp on one instead
I had to take this into account when I was taking measurements and drilling my holes in the bars, as everything needed to be shifted down by nearly two inches.
Once again, measuring a million times before doing any damage paid off!