Nurses at Beaumont Hospital in Dublin have served notice of industrial action.
It will start with a work to rule on June 10, and could escalate unless there is improvement in over-crowding and under-staffing in the Emergency Department.
The INMO said essential care will be provided for patients, but non-essential or administrative work will not be carried out.
Industrial action was deferred early this year, after agreement was reached on a package of measures to tackle overcrowding in the ED.
"We've had a lot of empty promises," said Lorraine Monaghan, industrial relations officer with the INMO.
"We've been through an LRC process in January this year, where management committed to recruit additional staff and fill vacancies that exist - and to put in place measures to address overcrowding.
"However, those measures have not been implemented - so there is a breakdown of trust."