The 17-year-sentence handed down to three Irishmen in Colombia last week was described as "a travesty of justice of international proportions" by Sinn Féin campaigners.
Party Assembly members Mr Gerry Kelly and Ms Caitriona Ruane flew to Colombia at the weekend to meet the men's lawyers.
The three - Niall Connolly, James Monaghan and Martin McCauley - were found not guilty at a public trial in April of training left-wing Farc rebels. They were found guilty of the lesser charge of travelling on false passports and released on bail until the Colombia Attorney General appealed against their acquittal on the main charge.
The men disappeared soon after being released and it is suspected they may have crossed the border into neighbouring Venezuela in June. They were not present when the 17-year sentences were passed last week.
Ms Ruane said Sinn Féin would be taking the men's case to the EU Parliament. She said they had been found not guilty at a public 10-month trial in front of the world's media and human rights organisations.
"Six-and-a-half months later, following an appeal by the Attorney General's office, we have a decision by three magistrates who met behind closed doors, who never spoke to any of the witnesses or defence lawyers.
"It is important to note that in this new appeal no new evidence was presented," she said.
After speaking with the men's lawyers, she said: "We are now exploring every legal option left open to the men. The lawyers are currently consulting experts in relation to this.
Fresh legal moves within Colombia are expected to be spelt out at a press conference by Ms Ruane, Mr Kelly and the lawyers in the Colombian capital, Bogotá, later today.
PA