The pair bought rights to 59,138 Irish Continental shares for as much as 25.20 euros apiece, they said in a statement on Friday.
Under Irish takeover rules, that increases their offer for the whole company to the same level, or 5 per cent more than the 24 euros bid last month by management group Aella.
The new offer extends a five-month takeover battle for Dublin-based Irish Continental, which operates ferries to the UK and France.
Aella made an initial approach in March and One51 and Doyle tabled a competing bid a month later. Irish Continental investors are due to vote on Aella's offer on Wednesday.
The ferry company's independent directors 'continue to recommend' Aella's offer and the shareholder meeting will still take place, they said in a statement.
Separately, property developer Liam Carroll has raised his stake in Irish Continental to 23 per cent, Dolmen Stockbrokers said on Friday.
Mr Carroll plans to block Aella's bid at the shareholder meeting, the Irish Times said on Thursday, without citing anyone.
The valuation of the One51-Doyle offer is calculated on the basis of 23.7 million shares outstanding and 1.81 million shares that may be issued under an option plan.