The Irish Nurses and Midwives Organisation (INMO) has said it will consult its members on their next move if pay or allowances are cut.
The INMO issued the warning as the Government examines public sector allowances as a whole.
Health Minister James Reilly will address the INMO's annual conference in Killarney later today.
General Secretary of the union Liam Doran said: "Any attempt to interfere with the rates of pay which are protected in the Croke park Agreement will be opposed by the INMO.
"We're always in contact with our colleague unions in the 24/7 alliance, that is the unions that represent gardaí, firemen, ambulance men and so on, and I've no doubt that we will do whatever's necessary to protect the rates of pay."