DUBLIN - MR Bertie Ahern tendered his resignation as Irish prime minister on Tuesday after 11 years in office, which made him one of Europe's longest-serving leaders.

Ahern handed in his seal of office to President Mary McAleese at her official residence on the outskirts of Dublin, after spending his final day in the post at the site of the historic Battle of the Boyne.

Elected Taoiseach (prime minister) in 1997, he will remain as head of the government formally until Wednesday, when his Finance Minister Brian Cowen is expected to be elected his successor in the Dail, or lower house of parliament.

In the state reception room at the president's official residence, Mr Ahern took around 20 seconds to sign his resignation letter and hand it to Ms McAleese.

The pair then retreated to state drawing room for a 30-minute meeting.

He waved goodbye to reporters as he left.

Before handing in his resignation, Mr Ahern said his contribution to a peace deal that largely ended three decades of violence in neighbouring Northern Ireland was his 'proudest achievement.'

'I thank God that we have closed the bloody chapters in our history and opened a new book of reconciliation and renewal,' Mr Ahern wrote in The Irish Times newspaper.

Mr Ahern also called for the Irish to vote 'Yes' to the European Union's key Lisbon Treaty in a June 12 referendum, which will provide Cowen with his first major test in office.

Ireland is the only EU member to hold a referendum on the treaty and the vote is being watched keenly across the 27-nation bloc as a rejection could scupper the treaty altogether.

'I am convinced that the ratification of the Lisbon Treaty is imperative to the future economic and social progress of the Irish people,' Mr Ahern wrote.

Hours before his formal resignation, Mr Ahern joined outgoing Northern Ireland First Minister Ian Paisley for the joint opening of the iconic Battle of the Boyne site north of Dublin in County Louth.

'Remember 1690'
At the 1690 battle Britain's Protestant king William III beat the deposed Roman Catholic king James II at the River Boyne west of Drogheda.

'Remember 1690' has since been a catch-cry for generations of Protestant loyalists in British-governed Northern Ireland.

The Boyne site has been developed by Mr Ahern's government as a reconciliation project, helping to forge a friendship between Mr Ahern and Mr Paisley and thaw relations between unionists and nationalists on the island.

Mr Paisley said there could be no turning back to the 'bad old days.'

'This must be the end of all atrocities and the building of the way to peace. The coming generation has a right to demand this and they must have it. We cannot fail them,' he said.

Mr Paisley, 82 - who is also stepping down later this month after decades at the turbulent frontline of Northern Irish politics - said he believed Mr Ahern would make a good Irish president.

'I think that probably he will be a candidate for the presidency,' Mr Paisley told RTE radio. 'After all he has done he deserves to be the president of the country.'

Mr Ahern won three successive general elections as head of a coalition government and oversaw an unprecedented decade-long economic boom, earning long-ailing Ireland the 'Celtic Tiger' moniker.

Along with then British prime minister Tony Blair, he helped seal the landmark 1998 Good Friday agreement for Northern Ireland.

The peace accord steered the province away from decades of violence between Protestants and Catholics and led to the restoration of a power-sharing administration in Belfast one year ago on Thursday.

Mr Ahern has said he will now fight to clear his name, saying when he announced his resignation he had 'never received a corrupt payment and I have never done anything to dishonour any office I have held'.

There is also speculation that he might run for the post of EU president - which would be created by the Lisbon Treaty.