The Health Service Executive (HSE) is investigating with the Food Safety Authority of Ireland (FSAI).
FSAI said the investigation is focusing on a catering company, Flanreil Food Services, but the source and cause has not yet been identified.
HSE said the sick people are from separate groups of family parties supplied by the North Dublin business on 13 May and 14 May.
Local media reported a woman has died but it is not confirmed if Salmonella was the cause of death.
Five people were admitted to hospital and 16 of those ill have been confirmed as being infected with Salmonella. The first cases were notified to the HSE on 18 May.
A Closure Order was served by the HSE on 19 May on the kitchen and associated storage facilities of Flanreil Food Services in O’Dwyers Public House, Strand Road, Portmarnock, Co. Dublin.
“The investigation is ongoing and includes further examination of the food business operation and food served and assessment of the information from ill and well persons who consumed food,” said HSE.
Symptoms come on within six to 72 hours following infection (generally 12-36 hours) and illness usually lasts four to seven days and most people recover without treatment.
All poultry farms in the Republic of Ireland are tested and monitored for Salmonella and any flocks found to be harbouring S. Enteritidis are slaughtered.
Most Salmonella can be killed by cooking to a core temperature of 75ºC instantaneously or an equivalent time temperature combination, e.g. 70ºC for two minutes.