It is remarkable that, out of such an uncertain beginning, one of the most durable arrangements in Irish politics has been forged. For over nine years Fianna Fail and PDs have led the country with Ahern and Harney as the two figureheads in the arrangement.

The ending of the AhernHarney axis is an issue for the government but it is far less of a problem than it would have been if the PD leader had departed earlier. If Harney had gone mid-term there might have been scope for her successor as PD leader to carve out different ground within government.

But as it is, this administration is on its final straight.

In seven or eight months' time the Dail will be dissolved and general election will be called. It would not be tenable for a new PD leader to cut loose now. The voters would hardly find such a development credible. If the two parties hung together during the Ray Burke resignation or the controversy over the appointment of former judge Hugh O'Flaherty to a top European job then it would need to be a big schism to allow the PDs to walk away now with any credibility.

This situation is especially true if Michael McDowell is the next PD leader. McDowell would continue as justice minister. Harney more than likely would stay in the department of health. To all intents and purposes, the only change would be a change in titles. McDowell would now be Tanaiste and PD leader.

The scenario might be different with Liz O'Donnell as PD leader. As a backbencher, the Dublin South TD has not been as closely associated with the post-2002 regime.

She might find it easier to create a different space for the PDs from their long-time government partners, but her room to manoeuvre would be similarly limited. There is simply not enough time left for a new PD leader to reorient the policy direction of the current government.

With policy stability more or less assured, the real threat to the government may come in the weeks ahead due to the personality change.

The relationship between Ahern and Harney has been vital to the coalition's success. Policy rows have been few enough, personality clashes even fewer.

By nature Ahern is a consensus politician. He ensured Harney was kept within the loop. There were regular meetings between the two leaders without the presence of officials or advisers.

McDowell would bring a different temperament to the relationship. But then again Ahern would not be working with somebody who was totally new to him. McDowell was attorney general for the second half of the 1997-2002 Fianna Fail/PD coalition. He's been at cabinet as a senior minister for the past four years.

McDowell would be the first PD leader not to have roots in Fianna Fail. But how he deals with the larger party in government may be what defines the early days of his leadership.

The Dublin South East TD would face considerable pressure to deliver success at the next election. His main choice in the months ahead would be to decide if the PDs should do as they did in 1997 and 2002 and contest the election as an independent party, or sign up to a pre-election policy agreement with Fianna Fail.

Last Thursday evening at a meeting in Dublin, McDowell was talking about the next election being a contest between "two competing alliances". That would see the PDs lining up solidly alongside Fianna Fail to counter the Fine Gael/Labour challenge.

Harney contested two elections as party leader, and if entering government is the definition of political success, then she was hugely successful. That is the benchmark against which McDowell, or any other new PD leader, will be judged.