Murphy, an alleged former IRA "chief of staff", was found guilty of tax evasion last week at the Republic of Ireland's non-jury Special Criminal Court.
Sinn Féin has called for the abolition of non-jury courts and praised Murphy's support for the peace process.
But Mr Robinson said a jury would have been "more susceptible to pressures".
Justice ministers in both the Republic of Ireland and Northern Ireland have also defended the use of non-jury courts which are often used to prevent potential jury intimidation in paramilitary-related cases.
The issue was brought to the fore on Sunday, when Sinn Féin leader Gerry Adams said: "All citizens have the right to be judged by a jury of their peers."
"It is extraordinary a case involving a failure to complete tax returns is heard before a non-jury court," Mr Adams added.
But speaking to the Dublin-based radio station, Newstalk, Mr Robinson said that he believed it was "an altogether appropriate type of hearing" for the Murphy trial.
"The idea that you could have had a jury trial in such a case, of course, is just sheer madness," the first minister said.
Mr Robinson also told the Irish state broadcaster, RTÉ, that he did not believe the outcome of the trial would have been any different if a jury had been used.
But he added: "I think a jury would have been much more susceptible to pressures."
Northern Ireland Deputy First Minister Martin McGuinness, of Sinn Féin, told Newstalk: "Tom Murphy, over the course of the last number of years, last number of decades, has been absolutely invaluable to building support for the peace process in south Armagh.
"So I think the work that was done by Tom Murphy in that area was good work and that makes him, in my opinion, a good republican."
Mr McGuinness told RTÉ that a "very unhelpful narrative" had developed over the case and claimed it was motivated by people who wanted to "undermine" Sinn Féin ahead of next year's elections.
Murphy, 66, lives on a farm in Hackballscross, County Louth, that straddles the border with Northern Ireland.
He had denied nine tax offences but will be sentenced in February.
It would have been "sheer madness" to set up a jury trial for prominent republican Thomas 'Slab' Murphy, First Minister Peter Robinson has said.