Fly-half Smith missed all three of England's matches in February after he got a calf injury in Girona before the tournament opener against Italy, while scrum-half Mitchell was unavailable last month due to a knee injury sustained in training after the win over Wales.
Their availability for the clash against title favourites Ireland will be seen as a major boost after England's crushing 30-21 defeat by Scotland.
Immanuel Feyi-Waboso, 21, who scored a try in Edinburgh last week, has also been included in the squad despite missing training so that he could sit an exam at his university for his medical studies.
Flanker Tom Pearson was not included in the squad, as well as scrum-half Harry Randall and centre Max Ojomoh being the other players released to allow for the return of Smith, Mitchell and Feyi-Waboso.
