Night buses operate all night, but if you want to get to the Hôtel Marignan at 13 rue du Sommerard (the cheap one, not the much fancier hotel with the same name) by train, two stops on the RER B and a seven-minute walk will get you there. I think the RER operates until 1am, and I'm sure the metro (don't call it the subway!) shuts down then.

Paris has interlinking metro and RER (regional) rail transit systems which are very efficient. I've grown to prefer taking the bus, now that smartphones have made finding a route so easy, but for a late-night arrival I'd stick with the RER.

Remember that France's time zone is one hour ahead of the UK's.