The side from the Principality went nearly ahead as early as the ninth minute when Valere Germain's rasping left-foot drive came back off the crossbar.
However, it was not until the 40th minute that Monaco took the lead when Yannick Ferreira-Carrasco's through ball played Germain again and, this time, he found the heart of the net.
Valenciennes didn't give up and were back on level terms with 20 minutes left when Arthur Masuaku's cross reached Tongo Hamed and his header was too much for Argentine keeper Sergio Romero as the already-relegated side look set to salvage some pride.
Their joy was short as Dirar struck three minutes from time when his left-footed shot found the top right corner of the goal and spared the blushes of the big-spending visitors.
The result did little for either side, with Valenciennes cast adrift in the Ligue 1 relegation zone and Monaco now guaranteed to finish second.