Air carrier FO reported a track deviation and restricted area incursion occurred while flying the RIVER visual approach to Runway 19 at DCA. The reporter cited confusion identifying the proper Runway as contributing; since the procedure lines up better with Runway 15 than with Runway 19.

Narrative

I was performing the duties of pilot monitoring for [our] flight to DCA. At that time the aircraft was on the approach to Runway 19 and assigned the RIVER Visual 19 approach to DCA. The Captain was performing the duties of pilot flying and was in control of the aircraft. The Captain and I had previously briefed the visual approach and backed it up with the LDA Z Runway 19 in the FMS. All checklists and flows were completed on time without any abnormalities. While flying the approach the Captain stated that he had the airport in sight; but not Runway 19. He also stated that it has been a while since I've done this" and asked me to help him identify the Runway. I informed him that the Runway straight off our nose was Runway 15 and if we continued to follow the river we would see the threshold and approach lights for Runway 19 to the left of it. The Captain stated several times "I don't have the Runway." While I continued to describe its location to aid in his identifying the proper Runway; I diverted my attention away from monitoring his course along the river. Additionally; the traffic departing the same Runway ahead of us was slow to start rolling so I was also monitoring for appropriate separation. When I noticed we had deviated left of the river he had identified the proper Runway and was already correcting. Despite the deviation; the approach was never unstable and continued to a landing without issue. I did not call a go-around for our lateral course because we remained on vertical profile. On the ground ATC provided a phone number for us to call the Command Center that monitors that prohibited airspace. The Command Center stated they received several radar hits inside P-56A at approximately XXXX feet and would be filing a report.Cause: I believe the cause of the lateral deviation into the prohibited airspace to be a result of a few compounding factors. The lack of experience in part; coupled with a Captain that had not done this approach in quite some time would be the first layer. Then the added complexity of a non-standard visual approach into complicated airspace while having minimum separation between us and the departing aircraft would be the next. The final contributing factor would be the added stress of the Captain's repeated commands to help him ID the Runway in a critical phase of flight. Suggestions: In the future this could have been avoided in a few ways. Most importantly; I should not have allowed myself to become distracted from my primary function of monitoring the aircraft and suggested the Captain go-around if he could not identify the Runway. Secondly; I could have called a go-around once I observed the diversion; though it may have only minimized it and not avoided it completely. Lastly; I should have been more assertive with my communications in the cockpit to ensure the safety of flight."