You can't really, LHD cars are perfectly legal here - just not practical, there are no obstacles to purchasing or registering them.
One third I think - but two of the most popular brands in Ireland - Toyota and Hyundai - are from RHD countries, so unless all countries globally agree to drive on the same side of the road as I said above, a development cost will always exist.
I know Sweden did this 50 years ago, but the world is pretty different these days. Most Irish drivers can't tell the difference between the driving lane and overtaking lane, so expecting them to not only learn how to drive on the opposite side of the road, but also learn to switch back to the other side when crossing into NI is just a horrible idea.