Ahead of the company's AGM tomorrow in Dublin, Pirc, an advisory group which represents investors with £1.5tn of assets under management, cited a string of concerns over the independence of O'Leary's fellow board members and recommended investors reject the airline's annual report.
In an alert issued this evening, Pirc said: "There is a lack of disclosure with respect of targets and measurable criteria for variable remuneration, which prevents shareholders from making an informed assessment."
It concluded: "[The company] could be overpaying [O'Leary] for underperformance."
It is not the first time Pirc has raised concerns about obscure targets and performance criteria used to set the controversial businessman's take-home pay. It issued similar warnings in 2015 and 2014.
Last year, 19 per cent of shareholders voted against the company's remuneration report in a mini-investor rebellion.
O'Leary, who has been chief executive of Ryanair since it floated in the early 2000s, owns 51 million shares in the company - around five per cent of the total. At today's closing price of €12.99 that gives the tycoon a paper fortune of €666m. He also holds options on another five million shares, which could be worth more than €20m in profit should he choose to cash them in.
Aside from raising concerns about O'Leary's pay, Pirc also attacked the independence of the nine non-executive directors that sit on the Ryanair board. They include former employees, big shareholders and, in one case, the deputy chairman of Ryanair's stockbrokers.
Pirc also said the practice of allowing the non-executives to take part in the firm's share option scheme was contradictory to standard governance rules, since it "may compromise their independence and lead them to focusing on the short-term."
The UK Corporate Governance Code advises companies pay non-executives a fixed sum so they can play their role as long-term stewards of the company's interests.