In Spain, the best way to book a reservation is to go to the station. There is no limited quota for pass users but trains can get full, so the best is to book one day or a few days in advance. You can do that at any ticket office in Spain, you don't have to do that at the departure station of the train.

Additional: this is the theory. In practice it seems to me that often you get simply waved off as they only speak spanish or dont want the trouble or simply do not even know how to handle this. Try again. BUT as most IR-users go in the main season=summer it must also be said that many long-distance trains, as they are so few in number, get booked out days advance then. Spain/Espana is simply the worst country to rely too much on IR. Also note that IR is NOT accepted on the brand new AVLO and Qui-GO low-budget fast trains MAD-BCN-but this might mean the normal AVE trains now have more space!