Mad-Mars: here is no through train, you have to do that in at least 2 stages and these cost the earth extra, if you are hell-bent on using the hi-speedo's. RYAN, if not on strike again, is likely cheaper as just these supllmts.
ONLY domestic in FR can be easily done online, from 2 or 3 month before, via oui.sncf.com. NOte: I've heard that in the UK this may lead to much higher priced other sites-read seat61 for advice. Its 10 eur per seat/trip if lucky, 20 if not (quota exhuasted), I've just printed out 1 myself.
INterna. for trains in+out of Fr is simply not possible online.
MUCH better advice and other many tips-how to travel far more cheaply etc. is on the dedicated forum of
www.rail.cc/en, run by IR-old hands peter and flo.
in general it can be said, that Es and Fr are simply the worst countries to use IR, plus that depending on route, savings may be nil compared with early advance fares.