A new secretary of state for N. Ireland is Hurd in Belfast

The British government's new overlord for Northern Ireland, Douglas Hurd, has had the shortest Ulster political honeymoon on record. He left his home in sedate Oxfordshire, snatched a quick lunch on an aircraft , and landed in Belfast to a predictably mixed welcome. Sinn Fein, the political wing of the illegal Irish Republican Army (IRA), dismissed him as a ''political lightweight.''

The Unionists (those favoring Ulster's union with Britain) voiced suspicion because Mr. Hurd once met Sinn Fein leaders during a briefing for a television program. Hurd, who takes over from James Prior as secretary of state for Northern Ireland, was also confronted with the immediate problems of a loyalist Protestant hunger strike against prison conditions, and by increased IRA violence in the province. Hurd will need all his skills as a politician and a former career diplomat to steer a course through a mine field.

He has the credentials, and the manner, of an urbane English gentleman. He was educated at Eton and Cambridge. In the 1950s he worked in the diplomatic service, with posts in Peking, the United Nations, and Rome. In 1966 he joined the Conservative Party's research department, and concentrated on foreign affairs. He is said to have impressed Margaret Thatcher with his grasp of European Community budgeting and Middle East diplomacy. And in 1979 Prime Minister Thatcher made him minister of state at the Foreign Office. In 1983 he moved to a similar post in the Home Office.

Mr. Hurd's knowledge of the complexities of Ulster is necessarily limited. He has visited the province only twice previously. Yet his diplomatic skill was immediately apparent upon his arrival in Belfast. He refused to be drawn by reporters' questions on security and closer cooperation with the Irish Republic.

He stated simply: ''A change of secretary of state clearly at this time and in these circumstances is not a change of policy. It is a change of person and it would be foolish for anyone to expect any dramatic changes from what my predecessor, Jim Prior, has been trying to do.''

Some people might conclude that Prior had courageously been trying to do the impossible - to help reconcile Ulster's estranged communities locked in a quarrel with historical roots reaching back hundreds of years. Mr. Prior set up the Northern Ireland Assembly as a forum for political dialogue, but it became a Unionist monologue since the mainly Roman Catholic Social Democratic and Labour Party, and Provisional Sinn Fein, refused to take their seats. They opted for Irish unity, thus leaving Prior high and dry, with his concept of shared government in the north.

Ironically, the electoral success of Sinn Fein, a byproduct of Prior's assembly elections, is a threat to both the Catholic and Protestant mainstream politicians.

Halfhearted talks took place between the sides during the summer, and Mr. Hurd's first task will be to try to salvage what he can from such tentative approaches. His other major test will be the run-up to the November meeting between Mrs. Thatcher and the Irish premier, Garret FitzGerald. Hurd will also be required to handle the political hot potato of British reaction to the New Ireland Forum report, a list of suggestions produced by Irish nationalists for possible Irish unity with Protestant consent.

In Dublin, politicians are pressing for a ''new initiative.'' Charles Haughey , the opposition leader, claimed that Catholic confidence in Ulster's police and legal system had evaporated, and he repeated his view that Ulster had failed as a ''political entity.'' His alternative of a united Ireland is unlikely to impress Mrs. Thatcher.

In real terms the best that Mr. Hurd might do is to improve relations with the Irish Republic, particularly on border security, keep as firm a hand as possible on terrorism within Northern Ireland, try to maintain and to create as many jobs as possible in an area of high unemployment, and attempt to edge the Ulster political parties even a few inches along the road of reconciliation.

It is a tall order, but Mr. Hurd has already set the tone with his careful approach. ''My first task is to listen - then to make statements,'' he says.

Thus far Mr. Hurd is giving nothing away, not even a hint of what he is really thinking. Those people with an eye for fiction might learn something from his writings. His main recreation is writing thrillers, and one of his eight novels is ''Vote to Kill.'' It deals with a Tory secretary of state who comes to Belfast with an ultimatum to the politicians - find agreement or the troops will be withdrawn.