Ireland scored four tries in the opening 35 minutes but made the game safe only in the final 10 after Italy came back into contention with a spirited display.
Farrell was unhappy with his team’s finishing in the 22, something he has lamented throughout this Six Nations campaign.
"The positive is the chances we created but the negative is that we did not take them all again. I don’t know what it was, four or five tries that we left out there," he told reporters.
"We let them into the game through lack of discipline at times. We did not control the game as much as we should do but test match rugby does not go your way the whole time.
"We found a way and it's nice to get the bonus-point win away from home. We still wanted it to be a better performance than it was."
Farrell had praise for Italy and said their improvement as a team under coach Kieran Crowley was obvious.
"All credit must go to Italy, it was a proper test match, they are a proper side," Farrell said. "Everyone can see the progression but when you are here and you are facing it... they were tough and resilient and in the forwards they gave us absolutely nothing.
"Defending their own line they were physical and in attack they cut us open from time to time."
Ireland travel to Scotland for their next fixture on March 12 with thoughts of a Grand Slam still alive.
"It is about the performance, about the next one. Murrayfield is a tough place to go. As is Rome," Farrell said.
