The Irish government regards the recent seizure of seven tons of arms and ammunition from the gun-running trawler Marita Ann by the Irish Navy off the southwest coast as a substantial victory in the struggle against terrorism, both in the Republic and in Northern Ireland.
Officials both here and in Belfast believe the arms, which included powerful modern machine guns, were destined for a stepped-up autumn campaign by the Provisional wing of the outlawed Irish Republican Army (IRA) in Northern Ireland.
They think this plan may have been foiled by the seizure. But more important, they hope it will have struck a major blow against the IRA sympathizers in the United States who provide the terrorist organization with the bulk of its arms and money.
Irish officials are hoping the US Federal Bureau of Investigation - using evidence provided by the Irish authorities - can trace the origin of the shipment and by doing so break up or at least seriously damage the IRA gun-running network. They also hope that in the course of this investigation evidence may emerge to implicate members of the New York-based Irish Northern Aid Committee (Noraid) who collect money for the IRA but who have denied involvement in this gun-running venture.
The Irish government is also pleased at the international cooperation which helped to provide the information that brought about the capture of the Marita Ann on Saturday. Both the American and British intelligence services are believed to have helped in the operation, and it is believed that an American satellite tracked the ''mother ship,'' registered in Canada, which brought the arms across the Atlantic.
The last dramatic seizure of a gun-running vessel in Irish waters occurred in 1973 when the motor vessel Claudia, carrying five tons of arms from Libya, was intercepted off the southeast coast.
The Libyan source of supply to the IRA dried up soon after, and since then the overwhelming bulk - perhaps 80 percent - of IRA arms has come from the US, which is also the organization's main source of funds.
However, the authorities know that shipments like these represent only a small proportion of the arms and ammunition brought in illegally over the past 14 years. Usually they are smuggled in cargo containers into ports in the Irish Republic and later transferred in small lots across the border into Northern Ireland.
Some of them never leave the Republic but find their way into the hands of terrorists or ordinary criminals here. In the past year, four members of the Republic's security forces, including two policemen and a soldier, have been murdered by the Provisional IRA or a breakaway terrorist gang. By a grim coincidence the fourth man, a chief prison officer (deputy warden) died on Saturday, the day of the arms seizure.
Irish taxpayers pay some $150 million to $200 million annually for security operations aimed at preventing the movement of terrorists and arms back and forth across the border. This is a substantial amount in a small country with a weak economy.
Within Northern Ireland, the nature of the IRA campaign has changed radically in the last year or two.
At present, it is scarcely aimed at all against the British Army or the local police, but is concentrated on killing Protestant farmers in (largely Roman Catholic) border areas.
The pretext is that these are usually part-time members of the security forces, but in reality the campaign is sectarian in motivation.
It is also aimed at provoking murders of Roman Catholics by members of Protestant terrorist gangs.
For many years, Irish politicians and diplomats - helped by Irish-American congressional leaders and, of recent years, Presidents Carter and Reagan - have tried to persuade Irish-Americans that contributing money to the IRA does not help to free part of Ireland from British rule. Instead, they say, it buys guns and bullets with which the Provisional IRA shoot other Irishmen.
But the flow of money from the US fluctuates according to the political situation in Northern Ireland. Upturns, in the words of a senior Irish official, ''reflect policy disasters by the British government.''
Before the hunger strikes by Provisional IRA prisoners in 1981, which claimed 10 lives, the American contribution may have fallen as low as $250,000.
The present figure is thought to be at least $2 million annually, and could be very much higher.
Officials here say that as long as guns and ammunition can be bought by criminals on the international arms market, and as long as American money is available, the campaign of the Irish Republican Army will continue in some form.