Anyone aged 18 who is refused service in a pub because it has an over-21s or over-23s policy can seek compensation under the Equal Status Act which came into force yesterday.
Under the new law, which outlaws discrimination in the market place, publicans can no longer insist on applying an age limit on service higher than the national drinking age of 18.
However, the Licensed Vinters' Association has described the age provision as "a bit stupid" in the absence of a national identity card scheme which would allow publicans to verify a customer's age.
The association, which represents about 650 pubs in the greater Dublin area, says publicans worried about the new law are now likely to insist on seeing valid proof of age, since they risk heavy penalties for serving under-age drinkers.
The Equal Status Act outlaws discrimination on grounds of age, gender, marital and family status, religion, race, sexual orientation, disability or membership of the Traveller community.
State and private goods and service providers, including banks, schools, restaurants and transport companies, are liable under the law.
However, publicans and other service providers can still refuse to serve someone if there is a substantial risk of criminal or disorderly conduct or damage to property. The new law also allows publicans to refuse in good faith to serve someone who does not look over 18.
The chief executive of the Licensed Vinters' Association, Mr Frank Fell, acknowledged that the new Act prohibits discrimination on specific grounds, but said "it is not a charter for everybody to come into a pub".
"If someone comes in and looks like they are going to break the place up, whether they are members of the Traveller community or not, they will be refused," he said.
The Minister for Justice, Mr O'Donoghue, signed the Equal Status Act into law yesterday.
The new law will be enforced by the Office of the Director of Equality Investigations, which can award compensation of up to £5,000.