Becoming a political leader is to a considerable degree a matter of luck. No matter what talents one may believe one has, they will get you nowhere unless there is a vacancy at the top - and, even then, all may depend on the competition for the job.
If Tom O'Higgins and Declan Costello had not been appointed to the judiciary in early 1977, and if either had sought the leadership of the party, I would not have stood for election to that position when, in July of that year, after the defeat of the national coalition, to everyone's surprise Liam Cosgrave resigned at the age of 57.
Retiring from the leadership of a political party also raises timing issues.
First, there is the matter of having the wisdom to give up that position before the party or the electorate starts to see one as a liability rather than as an asset. Not everyone judges that moment correctly! It is clear that Margaret Thatcher stayed too long for her own good. Tony Blair seems to have made the same mistake.
Éamon de Valera is the prime Irish example of someone who stayed too long as party leader.
As founder of Fianna Fáil, he was clearly reluctant to hand over the reins to a successor.
He could have retired from the leadership on or shortly after his defeat in the 1948 election, when he was 66.
But he was clearly not minded to do so, preferring to embark on a world tour to ventilate the partition issue.
Another opportunity occurred in 1952, when his party had returned to power and Seán T O'Kelly's first term of office as president was coming to a close. Many of de Valera's colleagues would have liked him to retire to the presidency of the State at that stage, and Seán T O'Kelly would presumably have stood down if asked to make way for the leader of his party. But nobody had the courage to suggest this to their founder/leader!
Total economic stagnation marked de Valera's last seven years as leader of his party - because all the chickens of his disastrous commitment to an inward-looking policy of self-sufficiency were coming home to roost.
After de Valera, no leader of either of the two main parties sought to retain party leadership beyond the age of 66. Charles Haughey remained until that age, but the typical retirement age of these leaders has been 61 or 62.
Most remained leaders for between 10 and 13 years, Haughey having been the longest-serving.
Mary Harney's 13 years as leader has been at the upper end of that range. Labour, however, has had two leaders who served respectively for 20 and 15 years - Brendan Corish and Dick Spring.
An important aspect of retirement from party leadership is the point in the electoral cycle at which a leader stands down. Fianna Fáil leaders have all retired in mid-term. Fine Gael leaders - who have experienced more electoral defeats! - have tended to retire immediately after losing office. This was true of James Dillon, Liam Cosgrave and myself, as well as Michael Noonan, more recently.
In 1978 I introduced to Irish politics a procedure where, in the aftermath of losing an election, the leader of Fine Gael must be the subject of a parliamentary party vote of confidence by secret ballot within one month. I felt this procedure would help avoid the temptation for party members to plot against a leader in whom they had lost confidence, but who nevertheless persisted in remaining on at the head of the party.
In March 1982 I was myself subject to such a vote, when five out of 88 votes were cast against me - which I felt was an astonishingly low figure, given that I had just lost office after only nine months in government!
In 1987 I decided to retire from the leadership of the party immediately after my replacement as taoiseach by Charles Haughey and, I think, I took my front bench and parliamentary party by surprise.
I then absented myself from the country until the date on which my successor was to be chosen, lest anyone might feel I would seek to influence the party's decision.
I know that some have felt I should have remained leader for a period after the election, but there were three reasons why I preferred to retire immediately after my defeat.
First - and least important - was the fact that when I was elected leader in 1977, I said I would serve for 10 years - and at that point in 1987 I had served for nine-and-three-quarter years.
Second, when the next general election would be due five years later I would be 66 and, in 1987, having just emerged exhausted from almost six very tough years in government, that seemed to me an advanced age at which to take on the job yet again.
(I could not then know that the incoming government would serve for only two years before calling another election, nor that I would in fact recover from my exhaustion and retain my vitality for many years thereafter!).
Third, I felt that once I had decided not to lead another government, I should not foreshorten my eventual successor's period of preparation for office by hanging around for part of the period between then and the next election.
I believe Mary Harney was right to retire now rather than wait until after the election. This will give Michael McDowell time to put his own stamp on his party, which has recently faced serious problems.
For even if most Progressive Democrats TDs survive the next election, which is not certain, a likely drop in Fianna Fáil's support and a corresponding recovery in that of Fine Gael means the present Fianna Fáil/PD alliance is unlikely to command sufficient votes to return to office without the support of Sinn Féin.
And Michael McDowell and his party will not accept participation in a government depending for its election and survival on that political party.
Given that Michael McDowell's right-wing and confrontational stance on many issues has alienated all the Opposition parties, if he wishes to continue in government after a possible Fianna Fáil defeat he may have some difficulty in becoming an acceptable member of an alternative rainbow government.
But he is a politician of great skill - so nothing should be ruled out!