Quinn (66) left the Training Unit of the north Dublin prison shortly after 9am and is understood to have been driven straight to his Co Cavan home.
He was sent to prison on November 2 after the High Court found him to be in contempt. He was released on three days' compassionate leave over Christmas to spend time with his family and attend a christening.
Today, a spokesperson for Sean Quinn support group Concerned Irish Citizens said: “Concerned Irish Citizens would like to welcome home Sean Quinn from Mountjoy, and hope that 2013 will be the year that the people of Ireland obtain answers to questions.
“The most vulnerable people in our society have no choice about austerity and brutal cuts so the least that they deserve is the truth, nothing more, nothing less.”
In November, Miss Justice Elizabeth Dunne told Quinn that his contempt was so serious that she could come to no other conclusion that it mandated a term of imprisonment.
She could not ignore the extent and degree of contempt by Quinn and imposed a nine-week jail term.
Quinn’s contempt related to allegations of a plan to put his family's €500m property empire beyond the reach of the former Anglo Irish Bank.
Judge Elizabeth Dunne ruled Quinn and his son Sean junior had attempted to put a multimillion-euro asset portfolio beyond the reach of the Irish Bank Resolution Corporation (IBRC).
Sean Quinn junior, who was also jailed for three months over the summer, has since offered to sell the penthouse apartment he shares with wife Karen on the outskirts of Dublin's Phoenix Park to demonstrate his willingness to purge contempt.
The bank, rebranded from the bust Anglo Irish Bank, is pursuing the family for debts of €2.8bn run up by Quinn senior in failed share deals.