The Government's been warned any legal action against Egypt over the treatment of Ibrahim Halawa could result in them pulling back from a commitment to release him.
The issue was raised in a briefing paper last month for the new foreign affairs minister, Simon Coveney.
In the document seen by The Irish Times says officials identified the Halawa case as one of three priorities.
It says any action taken in the International Court of Justice would be "complex" - and there would be no guarantee of success.
Speaking to 98FM News Somaia Halawa has dismissed the concerns, saying Egypt has failed to keep numerous promises relating to her brother's treatment. She has little faith in Cairo fulfilling its promise to release Ibrahim any event.
The Firhouse native has now spent over 1,400 days in prison since his arrest during demonstrations in August 2013.
The prosecution concluded its case against him at the weekend, as his trial was adjourned for the 26th time.