Wikipedia has the answer for some countries, see here: https://en.wikipedia.org/wiki/Right-_and_left-hand_traffic#Worldwide_distribution_by_country
It says Ireland is left, which is what I would expect as they drive on the left.
More countries have trains running on the left than drive on the left. Partially inertia, when cars switched to the right, trains stayed on the left as there was no advantage and big cost in changing (Italy, Portugal), partially because the British build the first railways (France).
As French trains run on the left, there is no crossover in the Channel Tunnel. But, as both Germany and Spain run on the right there are crossovers near the border. On High Speed lines they have to build a flyover to cross over, as crossing on the flat would cause too many delays.
Platforms can be on either side, at the sides of the two tracks or between them.
This of course only applies to double tracks, not single tracks. And around stations it is all complicated as trains cross over. And 4-track routes can have more complications.
It doesn't make much difference to which side you sit, it only affects which side trains coming in the other direction pass.