Harry Reid: As the people of Ireland go the polls today, it is salutary to realise that their country has a considerable diplomatic and constitutional stake in the United Kingdom.

As the people of Ireland go the polls today, it is salutary to realise that their country has a considerable diplomatic and constitutional stake in the United Kingdom. This is a direct result of the Anglo-Irish Agreement of 1985, which gave the Dublin government, and in particular its leader, the Taoiseach, the right to play a major role in negotiations on the future of Northern Ireland. In other words, Dublin was granted an official right to engage in one of the most sensitive ongoing problems in what was, and is, a foreign country.

For the past 10 years the Taoiseach has been Bertie Ahern, one of the most durable and popular of Europe's leading politicians. He has played a key role as the Northern Irish peace process has progressed in its tortuous way to the current happy outcome.

His contribution has been less trumpeted than Tony Blair's, but it has been equally important.

Ahern's role in bringing peace to Northern Ireland is by no means the key issue in today's Irish election, which will be fought on more domestic issues. But it might yet prove of some benefit to him as he faces the strong challenge of Enda Kenny, the affable leader of the main opposition party, Fine Gael.

These days there is no suggestion that the national integrity of either the UK or the Republic of Ireland has been vitiated or damaged by the Anglo-Irish Agreement. Quite the contrary; both countries have emerged the stronger for it. This illustrates in the very best way the importance of the idea of inter-dependency, a word usually used in the contexts of the environment, trade, energy supplies and so on.

It is rarely mentioned in a constitutional context. And yet the huge significance of the Anglo-Irish Agreement of 1985 is that two countries, until then often wracked by mutual suspicion and dislike, started to work together constructively. What was needed was the bold breaking of the barriers of national pride and tradition.

The early signs were not propitious. It took considerable political courage for Margaret Thatcher, the main architect of the agreement, to face down the Ulster Unionists, whom she had regarded naively as natural allies. The response of Ian Paisley, now the leader of Northern Ireland and a man happy to be seen basking in his convivial friendship with the Taoiseach, was visceral. He denounced Thatcher violently from his pulpit as a Jezebel. He accused her of capitulating to the IRA and seeking to destroy Northern Ireland.

Other leading Unionist politicians were even more insulting. They called her a traitor and a prostitute. Her closest political ally, Ian Gow, resigned from her government because of what he regarded as the unacceptable involvement of Dublin in the affairs of the UK. Gow was murdered by the IRA five years later.

In Scotland, the ramifications of Thatcher's far-sighted and brave policy were less dramatic, but still very damaging to her party. The Orange Order formed the Scottish Unionist Party as a direct result of the agreement.

The SUP did not put forward candidates at the 1987 General Election but it encouraged its supporters, who were stridently opposed to the agreement, to punish the Tories in Scotland. This contributed to the disastrous performance of the Scottish Tories, in the west of Scotland in particular.

Of all Thatcher's controversial innovations, the most hated and vilified was not the poll tax but the agreement of 1985. Yet it has proved to be a visionary agreement that has been enormously beneficial, not least to the very people who denounced it so viciously.

Thatcher enjoyed good relations with Garret Fitzgerald, who was Taoiseach in 1985. Two years later she had to work with the new Irish leader, Charlie Haughey. She found him a tricky man to do business with, and reckoned he was unco-operative on security matters.

Yet the agreement survived, and for a decade Bertie Ahern has been one of its most charismatic and positive exponents.

One of the ironies of today's election in Ireland is that Sinn Fein is in a less strong position in the Republic than it is in Northern Ireland. In the previous Irish election, in 2002, Sinn Fein gained just 7% of the popular vote. Polls suggest this figure could go up slightly today, but not dramatically. More voters have indicated that they would not object to Sinn Fein sharing government, as in Northern Ireland, but Ahern has stated that he will do no deals with them.

After 10 years in power, Ahern stands on the brink of an unprecedented third term.

Meanwhile, Tony Blair is bowing out, tainted and disgraced by the debacle of Iraq. Like Blair, Ahern has been a constructive peace-builder in Northern Ireland. Unlike Blair, he has not unleashed war elsewhere in the world.