The full back renowned for scoring the goal which guaranteed Hoops' passage to the Europa League is back at the Tallaght Stadium
The right back has rejoined the club he left during the close season following a period out of the game in Australia.
Sullivan will be eligible for league action once the Airtricity Premier Division recommences after the summer break as well as Stephen Kenny's side's European campaign.
Twenty-nine year old Sullivan won the League of Ireland Player of the Year award in 2011 after scoring the all-important away goal against Partizan Belgrade.
His strike helped Rovers secure the 2-1 win they needed away from home in order to become the first-ever Irish side to play in the Europa League group stages.