Oil exploration company Providence has said the Barryroe oil field off the southern Cork coast probably contains over 1bn barrels of oil.
There is a possibility that it could contain up to 1.6bn barrels.
In a statement to the Stock Exchange, the company said it was up to four times previous expectations.
The assessment was based on data from six oil wells drilled by Barryroe, together with 3D seismic data, as well as other regional data.
Providence is listed on the Dublin's ESM and the AIM in London.
The Chief Executive of Providence Resources has said work will now commence on figuring out how much oil can be extracted economically from the Barryroe oil field.
Tony O'Reilly Jnr said new plans and maps would be drawn up to see how the recovery could be maximised as cost effectively as possible.
Providence is to start looking for partners to continue the project through to first production.
Mr O'Reilly said the quality of oil tested was better than expected.
Analysis had shown it to be a "sweet, light crude that moves well through the reservoir", which he likened to "a full bodied claret".
'Major oil field, even by North Sea standards'
Mr O'Reilly described Barryroe as a major oil field, even by North Sea standards.
However, he stressed that the estimated 1bn barrels of oil in the field should be described as "oil in place".
"You obviously have to then look at the abstraction technology to see exactly how much of that you can get out of the ground," Mr O'Reilly said.
"The key industry metric that everyone looks at is what is the recovery rate, we haven't put out any formal recovery rates yet, but to give you some guidance in the North Sea the average recovery rate from oil fields is about 38%."
"It may be a little bit less, it may be more, it really depends on the level of investment you make and the methodology you use to extract."
In relation to any potential concerns by local communities about the extraction process, Mr O'Reilly said that one of the fundamental differences about the future Barryroe development is that it is offshore.
Mr O'Reilly said local concerns are something that a company has to be aware of.