I have just become eligible for a senior railcard. This question is more in hope than expectation: can I get any discounts outside the UK?
I guess that the answer is no. The FAQ page tells me that it is not valid in the Republic of Ireland or even Northern Ireland so it is not actually even a whole UK railcard. This strongly suggests but does not confirm that it has no value beyond.
Addition: I ask with a little hope because some England / GB / UK cards have some value beyond their primary territory e.g. National Trust and English Heritage.
Addition: Use as proof of age is not interesting as, if I am overseas, I will necessarily have something else which is likely to be accepted e.g. my passport or national ID card. Based on experience with some other memberships, I had hoped that a few other organizations might accept it. I gave a couple of examples in my previous addition: National Trust and English Heritage cards are accepted in some places beyond their primary territory. My Oxford Union membership card allows me access to a variety of organizations around the world.