Defeat in Dublin prevented England recording back-to-back Grand Slams in the RBS Six Nations and ended their 18-match unbeaten run.
England captain Hartley told ITV: "Big lessons to learn today. We set out to win the tournament and we've done that.
"Obviously disappointed not to win this final game because we had high hopes, we had high expectations of ourselves.
"It goes to show we're not quite there yet as a team. Full credit to Ireland, (they) put us under unbelievable pressure today and gave us one hell of a Test match.
"We're not the finished article, this will keep us grounded, plenty of work to do. But full credit to Ireland today and this atmosphere was a great one to play in."
On England's performance, Hartley added: "I think it was negative upon negative. We seemed to back every error up with another error and they executed well."