Man-of-the-match D'Arcy scored one of Ireland's eight tries that gave them an excellent chance of winning their first Six Nations title since 1985.
"We knew we had to score tries," the Leinster centre told the BBC.
"We put Italy under the screw from the word go, we had to not make mistakes and kick all our penalties.
Watson helps England to easy victory over Italy
"But we were disappointed to let them in for two tries at the end.
"The crowd were great though, we've always said the Irish fans are some of the best in the world."
Girvan Dempsey and Denis Hickie both grabbed two tries, while Shane Horgan, Ronan O'Gara and flanker Guy Easterby also crossed for the Irish.
Italy's tries came in the final 10 minutes and were scored by skipper Marco Bortolami and full-back Roland de Marigny.
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