LONDON — A POLITICAL scandal dating back eight years has made it highly likely that the Irish Republic will get its first woman head of state when voters go to the polls on Wednesday. Mary Robinson, a crusader against social injustice, with Labour Party backing, shot ahead to become front runner in the presidential race when it emerged last week that Brian Lenihan, candidate of the ruling Fianna Fail Party and the Irish Republic's deputy prime minister, had lied about his part in a government crisis in 1982.
The revelations persuaded Irish Prime Minister Charles Haughey, who heads a coalition government, to sack his deputy. By doing so, he narrowly escaped losing a vote of confidence in the parliament last Wednesday, but by then Mr. Lenihan's prospects had plummeted. From being 17 points ahead of Mrs. Robinson, he found himself trailing the Dublin lawyer by 21 points.
The two candidates are both Roman Catholics, but Robinson has a radical record on issues on which Lenihan rates as a conservative. She has campaigned against Ireland's constitutional ban on divorce and argued that restrictions against contraception and homosexuality be eased.
A third candidate for the presidency, Austin Currie of the opposition Fine Gael party, was not expected to gain more than 20 percent of the vote.
The scandal that propelled Robinson into an apparently commanding position appears to have weakened the position of Mr. Haughey, Fianna Fail's long-time leader. He depends for a parliamentary majority on support from the small breakaway Progressive Democratic Party. Members of that party had demanded Lenihan's resignation as the price of their continuing support. Haughey won the vote of confidence by a margin of three.
The 1982 scandal concerned telephone calls made by Lenihan to President Patrick Hillery, asking him to appoint a Fianna Fail government led by Haughey, without an intervening general election. Such an approach was unconstitutional. Lenihan's presidential aspirations began to take a tumble last month, when he denied having sought to influence Dr. Hillery, but then had to admit that he had told a journalist of calls he had made to the head of state.
If Robinson is elected, voters can expect their country's presidency to undergo a drastic change of style.
Her campaign slogan is ``President with a Purpose,'' and she has said that she will turn what has been mainly a ceremonial office into a pulpit in a crusade against social injustice.