Shares in Enterprise Oil slipped in late-morning trade as investors began to take notice of the production warning in the company's first-half trading statement.
Shares in the company, which operates the Corrib gas field off Mayo, lost about 6 pence, but the losses soon escalated as more investors noticed the group's reduction in its production target.
In March, the group predicted production would be in the range of 250,000 to 260,000 barrels of oil equivalent per day (boepd).
Today the group said: "Production will average just below the range published in March," but the comment was buried on page six of the statement, and it took a while for some investors to notice it.
The news offset a better-than-expected first-half post-tax profit figure. For the six months to June 30th, Enterprise reported a post-tax profit of £204 million sterling. Analysts had been looking for a figure between £170 and £190 million.
The company said the Corrib development continues to gather momentum. The development is expected to ensure an indigenous gas supply in Ireland for at least 15 years.
Planning permission for an onshore gas terminal - subject to an appeal to be heard later this year - was granted in August by Mayo County Council.
Agreement has been reached for the sale of 60 per cent of the group's Corrib gas to Bord Gáis, and the group is discussing the sale of the rest of its share with other potential customers.
Additional reporting from AFP