Irish Taoiseach Brian Cowen has re-confirmed that "the 'guarantees' promised by EU leaders at the December summit 'must be legally robust in order to reassure the public about the treaty,'" recalls Crossick.
The plan, which was announced by French President Nicolas Sarkozy, is intended to provide Irish voters with legal guarantees before a second referendum in the autumn.
"The question arises, however, whether it is legally possible to add to an accession treaty an Irish-specific protocol containing such legal guarantees, or whether only issues pertaining to a state's accession may be dealt with," writes Crossick.
Cowen has lobbied EU leaders for legal guarantees on Lisbon, specifically regarding having social and ethical issues, neutrality and taxation incorporated into the existing EU treaties at the earliest possible opportunity.
As Croatia is expected to join the EU in 2010-11, the idea is to add a protocol to its accession treaty.
Crossick quotes MEP Andrew Duff as saying: "Adding this protocol to the Croatian accession treaty would leave the treaty wide open to attack in the courts."
"If an accession treaty is not used, the legal implementation would have to await a new EU treaty to be drawn up and ratified," writes Crossick.
"The objection raised is fascinating for lawyers, but may not have any political or practical relevance," he adds.
"Even if someone with locus standi were willing to bring a case, it is unlikely that the ECJ would strike the provisions down," Crossick concludes.