Providence Resources' chief executive Tony O'Reilly has stepped down from the position and has resigned from the board with immediate effect.
Mr O'Reilly has been CEO of the exploration company for more than two decades and the company said he was leaving as part of a "mutual and amicable agreement".
The company said it had started a process to recruit a new CEO to lead Providence in its next phase of development.
A further announcement will be made as soon as this process is completed, it added.
Providence said that Mr O'Reilly has agreed to work with the board until the end of January 2020 to ensure an orderly transfer of his responsibilities.
In the interim, his executive functions will be assumed on a temporary basis by Chairman Pat Plunkett.
Tony O'Reilly said that after more than two decades with Providence, it was time for him to pursue new opportunities.
"I am extremely proud of all that we have achieved over the years and the key role that our collective team efforts at Providence have played in establishing interest in Ireland's offshore arena," the outgoing CEO said.
"I wish all stakeholders in Providence every success in the years ahead, particularly with the Barryroe project," he added.
In October, Providence Resources failed to received $9m in expected funding from Chinese investor APEC Energy Enterprises.
The funds had been intended to cover the exploration costs at the firm's Barryroe site off the Cork coast.
Mr O'Reilly said he had staked his reputation on the issue of the Chinese funding, which initially had been due in June and the deadline for which had been extended on several occasions.
Earlier this year, Providence also announced a redundancy programme where its technical and support staff will be made redundant as part of the re-engineering of its business model.
It also voluntarily surrendered the frontier exploration licence for the Diablo prospect.
Providence's Barryroe project is the most advanced and appraised oil project offshore Ireland, but it still needs to be appraised by additional drilling before major capital commitment to its development will be made.
Providence Chairman Pat Plunkett said that since the foundation of the company, Mr O'Reilly has been the main driver behind the development of the business.
The chairman said that Mr O'Reilly had been passionate in promoting the company's role in oil and gas exploration in the Irish offshore sector.
He also led the company into partnerships with some of the world's leading energy companies including ExxonMobil, Eni, Chevron, Repsol, Petronas and Total.
"A major milestone during his tenure at Providence was the successful drilling and testing of Barryroe located in the North Celtic Sea Basin which has provided the company with a world class development asset which is capable of providing significant shareholder value in the coming years," Mr Plunkett said.
"Tony leaves by mutual and amicable agreement and has the great appreciation of the board for his management of Providence through multiple E&P cycles," the chairman said.
"On behalf of all of Providence's stakeholders, I would like to thank Tony for his many years of dedication and hard work at Providence and I wish Tony every success in his future endeavours," he added.
In a note, Davy Stockbrokers said that Mr O'Reilly's departure was a "milestone event" for Providence Resources.
Davy said the new CEO, when appointed, will have to pursue the group's Irish offshore assets, chief among which is the requirement to progress the appraisal of the Barryroe project in the Celtic Sea.
The stockbrokers said the difficulty and twists and turns surrounding the efforts to put the Barryroe appraisal programme in place conceal the fact that it is a substantial target, where oil has been identified in numerous wells.
"It needs further appraisal, but its potential rivals and is larger than most mature late-stage North Sea projects (depending on the development route chosen, the scale of the project is sufficient to model targets of several hundred million barrels)," they added.
Barryroe sits in relatively shallow water in a stable political environment and has attractive fiscal terms, Davy said.
It can also be developed in a controllable staged fashion with an option for early production to be put in place, the stockbrokers added.