Talks are to resume today at the Labour Relations Commission in an effort to find a resolution of the Irish Ferries dispute.
Management of the company held talks with Siptu yesterday after both sides agreed to a recommendation by the National Implementation Body (NIB) to attend talks.
Siptu is involved in a bitter dispute with the company over its plans to replace hundreds of Irish staff with cheaper labour from eastern Europe.
However, a potential deal is under threat today because of continued disagreement over the company's plan to remove its ships from the Irish register.
The union claims the only way to ensure the company adheres to Irish employment laws, including the minimum wage, is by preventing it from "re-flagging" its vessels to another state. Irish Ferries has applied to re-register its ships in Cyprus.
A compromise formula proposed by the NIB would have allowed the company to re-flag its ships once it had entered a binding agreement to maintain Irish employment standards.
Siptu's marine branch organiser Paul Smyth described this proposal last night as a "pipe dream" because, he claimed, it would be unenforceable.
Meanwhile, union leaders are to decide today whether to go ahead with a national protest on Friday in support of workers at Irish Ferries
Siptu said yesterday the event would go ahead as planned despite the start of the LRC talks. Other unions are understood to believe it should be called off if the Irish Ferries dispute is resolved in the meantime.
The executive council of the Irish Congress of Trade Unions is expected to discuss the dispute later today.