Members of the Psychiatric Nurses Association are balloting for industrial action.
It is in response to what they say was a "unilateral decision" by management of St Patrick's Hospital to close the staff's defined benefit pension scheme from this June.
Nurses also staged a second protest outside the hospital in response to the decision to close the scheme.
The union says the closure will impact on 274 active members and 224 deferred members of the scheme.
PNA General Secretary Peter Hughes said the decision reneges on an agreement reached with the hospital, the pensions board and trustees.
Meanwhile, SIPTU members at St Patrick's Hospital have also begun a ballot for industrial action over the pension issue.
SIPTU Organiser Aideen Carbery said in a statement that the ballot "results from a decision by senior management and the Board of Governors of St Patrick's Mental Health Services to cease employer contributions to the staff defined benefit pension scheme".