An A320 entered IAD Runway 19C after being told to hold short because the runway markings did not clearly indicate that the W2 intersection was an entry to Runway 19C.

Narrative

We landed on Runaway 19R. Tower instructed us to taxi via Taxiway W2 and to hold short of Runway 19C. Runway 19C was closed for construction and men and equipment were located on it. The Captain taxied onto Runway 19C without holding short; and then stopped when he realized he was on the closed runway. He asked me if I thought we had taxied onto Runway 19C and I replied that I did; although I pointed out that I had not seen the typical runway signage prior to our movement onto the closed runway. There was a fair amount of construction and it appeared that that intersection was either poorly marked or not marked; or we both failed to see the markings. At that point the Tower reminded us that we had been instructed to hold short. I apologized and we continued to our gate without further incident. The following aircraft that landed on Runway 19R (another major Part 121 carrier) offered an unsolicited remark to the Tower Controller that he could see why we had failed to hold short; since the intersection/runway were poorly marked.