The action is in protest at a decision to cut 169 jobs in the Western Health Board area.
The 40 clerks, who are members of SIPTU, will initially refuse to answer telephones. They will decide on any further action later this week.
The clerks say they are determined to fight the Health Board's attempts to save €1m by laying off 169 clerical and administrative workers.
Other SIPTU members employed at the hospital will meet this evening amid growing calls for all-out industrial action.
SIPTU's Galway Branch Secretary says they have been reluctantly forced to take this action. Colm Keaveney says their aim is to highlight the growing seriousness of the situation and to protest at the Health Board's refusal to take part in what he describes as the normal consultative process.
Meeting next Thursday A Western Health Board spokesperson told RTÉ News that a meeting had been arranged with union representatives for next Thursday. She said they found it "unusual" that a ballot for industrial action had taken place before that meeting.
The ward clerks staged a token protest at the hospital entrance this afternoon after they had voted unanimously for industrial action.
SIPTU says it will escalate the industrial action unless the Board reverses the decision to lay off the 169 health care workers.
IMPACT nationwide campaign of resistance IMPACT General Secretary, Kevin Callinan, said his union will mount a nationwide campaign of resistance to the planned health cuts.
IMPACT has ordered its members not to provide cover for staff who are to be laid off in any of the country's health boards.
The National Executive of IMPACT- which represents 25,000 health care workers nationwide - is meeting in Dublin today to decide on its strategy to oppose Government cuts in health spending.
IMPACT is to have a further meeting with executives of the Western Health Board in Galway tomorrow.
