O’Brien, aged 52, of Kilmore, Monkstown Grove, Co Dublin, faced 19 charges of theft and 19 charges of deception from five individuals.
A further seven charges of deception involving €1.9m relate to one of those five people between 2003 and 2008.
O’Brien had denied all charges, but changed his plea to guilty on the day his trial was due to start.
He pleaded guilty at Dublin Circuit Criminal Court to 14 sample counts representing the total amount of charges.
They included stealing money and dishonestly inducing people to invest in bogus shipping and insurance schemes, and also bogus property schemes in Paris, Manchester and Hamburg. The allegations of deception involve five people, Martin O’Brien, Pat Doyle, Evan Newall, Louis Dowley and Daniel Maher.
It was alleged that about €4m was stolen from Mr Dowley, a Tipperary dairy farmer.
O’Brien also admitted stealing €2.1m from Evan Newall between June 8-15, 2007, at National Irish Bank, Stillorgan, Co Dublin.
Other offences took place at NIB Donnybrook and Ulster Bank, Donnybrook, Dublin 4.
Luán Ó Braonáin, prosecuting, said O’Brien’s plea was acceptable to the DPP.
Patrick McGrath, defending, said his client has no previous convictions.
In May 2013, O’Brien lost a High Court judicial review challenge arguing he could not get a fair trial due to adverse publicity in the print and broadcast media. He appealed the outcome to the Supreme Court, which ruled that the trial could proceed any time after June 2014.
Earlier this month, O’Brien made a separate application to bring a fresh judicial review challenge, on grounds arising from a recent Supreme Court ruling concerning rights of detained persons to have solicitors present during interviews.
Judge Patricia Ryan adjourned the case for mention on June 25, when a date for sentencing will be set. O’Brien was remanded on continuing bail.