Any prosecution of terrorist offences against Dundalk woman Lisa Smith may hinge on whether "theatre of war" evidence is admissible in court, how that evidence was gathered and who collected it.
Ms Smith is being questioned for another 24 hours by detectives in Dublin following her arrest on a flight from Turkey on Sunday.
Building a case against the former Air Corps crew member to charge her with the lesser offence of membership of an unlawful organisation is likely to be the preferred route pursued by the State.
This type of case – should the Director of Public Prosecutions proceed with it – might not be straightforward.
Ms Smith's solicitor, Belfast-based Darragh Mackin of Phoenix Law, has already pitched a defence that seeks to undermine any accusation that she was a member of the terrorist organisation known as Isis.
He claims that in 2015 she travelled to the "caliphate" that Isis created out of large swathes of Iraq and Syria it controlled out of a sense of calling as a devout Muslim to a "euphoric land".
She has denied any involvement in any terrorist activity.
Detectives are investigating Ms Smith for terrorist offences under the 2005 Criminal Justice (Terrorist Offences) Act, examining whether she was a member of an unlawful organisation – an offence which carries a sentence of up to seven years – or whether she was involved in terrorist activity, which carries up a maximum sentence of 10 years.
“In most of the countries, membership of a terrorist organisation is the most common approach to prosecute these kind of people who have gone and joined Isis,” said Tanya Mehra, a researcher at the International Centre for Counter-Terrorism at The Hague.
“Proving membership of a terrorist organisation is slightly easier than proving any actual acts they are alleged to have committed.”
Relying on evidence gathered by a foreign intelligence service to support either charge raises complications around how it was collected and managed in order to present it as evidence in an Irish prosecution in court.
That service may not wish to reveal in open court the intelligence-gathering methods that would tie an individual to a suspected terrorist offence in the Islamic State, which could undermine the evidence. Defence lawyers could also seek to discredit intelligence produced by American or Iraqi security forces over past practices including torture.
In 2017, two Iraqi twins were acquitted by a Finnish court of charges related to a massacre in Iraq because their defence successfully raised doubts about images from an Isis propaganda video and the veracity of Iraqi documents.
“Any defence worth its salt would immediately use that as grounds to go before a judge and say your honour this is completely inadmissible,” said Raffaello Pantucci, associate fellow at the International Centre for the Study of Radicalisation at King’s College, London.
UK Home Office statistics show the challenges of taking successful criminal cases against British returnees from the Islamic State. Only one in 10 people “of national security concern” who returned from Syria have been prosecuted.
Imran Awan, a criminology professor at Birmingham City University and an expert on Islamophobia, said the number was small "purely because of the very difficult evidential burden of proving that people have gone out to fight".
He said the UK has chosen to avoid the court route by stripping suspected Isis supporters of British citizenship, as it did in the high-profile case of British-born Shamima Begum who left London to join Isis in 2015 when she was 15.
Mr Pantucci has rejected Ms Smith’s defence as not credible.
“I don’t know if I buy this sort of babe in the woods who thought they were building the perfect Islamic state,” he said, noting that the Dundalk woman appeared “fairly educated”.
“They were building what they interpreted as a perfect Islamic state but it is one that has all sorts of murderous consequences that are very hard to miss.”