And although centre McSharry and Niall O'Connor's boot pulled the hosts back into contention, they ran out of time and had to settle for a losing bonus point.
Connacht were looking to arrest a slide of seven straight defeats in all competitions and started the game with some great attacking intent.
Some great passing and linebreaks eventually led to a penalty for Niall O'Connor on four minutes but the fly-half could not master the conditions nor the angle.
Connacht continued to press and looked like the more likely side to get on the scoreboard first but on 11 minutes Treviso were awarded a penalty for a ruck infringement and De Waal made no mistake with his kick.
And it got worse just four minutes later as early of Connacht's early promise was blown away as Brendan Williams touched down for the Italians after good support work by Ludovicio Nitoglia , De Waal converting for a 10-0 lead.
But the mood around the Sportsground worsened after 23 minutes as wing Iannone went over for a try after winning a footrace to a loose ball, this time De Waal missing the conversion.
The home side were continuing to press and Treviso scrum-half Fabio Semenzato soon found himself in the sin-bin.
And the Irish side soon made the most of their man advantage with their only try of the game as centre McSharry helped himself to his first try in Connacht colours by barrelling through several Treviso defenders, O'Connor this time successful with his kick.
Connacht had their first opportunity to reduce the arrears in the second-half eight minutes after the restart but O'Connor pulled his penalty from the Treviso ten metre line wide of the posts.
And the home side were made to wait until the 57 minute to get on the scoreboard again, Treviso flanker Monoa Vasawai sent to the sin bin and O'Connor making no mistake from the resulting penalty to bring them to within five points of their Italian visitors.
Connacht were doing their best to cross for their second try of the game but just as it looked as though captain John Muldoon would find his way over, the referee blew for a penalty and they had to settle for an O'Connor penalty.
And although they continued to press they were unable to find another score and have now tasted defeat in their last eight games.
|Connacht Rugby Score Card|
|Benetton Treviso Score Card|
|Willem de Waal||1||1||5|