IRA's Bomb Shakes N. Ireland Catholics

'THEY'RE Back You Know'' - these words painted in two-foot-high lettering on a wall in Belfast heralded the end of the Irish Republican Army's cease-fire.

After 17 months, the fragile peace was shattered with a bomb in a London office block. And back here in the capital of Northern Ireland, graffiti-artists were quick to record the return to violence as the end of the cease-fire sent shock waves rippling through both Catholic and Protestant communities.

Yet substantial sections of the population have already expressed a determination to restart the peace process even as British soldiers return to the streets of Northern Ireland in full combat gear, and police in flak jackets set up road-blocks and checkpoints.

Peace rallies in Belfast and Londonderry, as well as public meetings, are planned so that the so-called silent majority in favor of peace can make their feelings known.

Further, the blast has created a divide among Catholic nationalists, who seek Northern Ireland's independence from Britain, but disagree over the IRA's decision to return to violence. Many, such as community worker Donncha MacNiallais, blame the British government.

''Sinn Fein [the political wing of the IRA] did a good job in keeping the IRA on track for peace, but it seems to me that in the end it was obvious the British were not going to do anything to help keep the peace,'' she says.

Others, however, in the heart of Londonderry, where support for the IRA has traditionally been strong, say the organization had other options besides violence.

''No one can understand why this happened now,'' a young shop worker says. ''There were other times when the peace process seemed to be going nowhere, and nobody would have blamed the IRA for giving up on talks, but not now.

''I think, and I know a lot of other young people feel the same, that the IRA has played right into the hands of the unionists,'' he says, referring to the largely Protestant community that favors maintaining British rule in Northern Ireland. ''Protestants said all along that the IRA would return to violence, and they were proved right.''

Indeed, Kathleen Ferguson, whose policeman son was killed by the IRA while on patrol in Londonderry, says she has never believed the cease-fire was permanent.

''If the IRA had meant to stop the killing forever, they would have handed over their weapons. That was all that was stopping the all-party talks from going ahead. If they really wanted to negotiate, the arms would have been given up.''

Protestant political leaders also say the blast shows that the IRA never intended to lay down their arms for good.

''Unionists weren't fooled for a minute,'' says Gregory Campbell, spokesman for the Democratic Unionist Party. ''The British government seemed to be for a while, but now they know the truth, and what we want now is a complete clampdown.''

''We want security stepped up immediately. Sinn Fein and the IRA have shown themselves in their true colors,'' he adds. ''Protestants are very angry, and we want the full force of the law brought against them so that they can be wiped out completely.''

Only time will tell whether the London bomb was the start of a full-scale return to violence by the IRA or an attempt to force the British government into reaching agreement with nationalists.

For the moment, an uneasy gloom has descended on Northern Ireland. Few people want a return to security checkpoints on every corner and the constant threat of bombs and bullets.

The hope is the ''They're Back You Know'' graffiti artists will soon change his slogan to ''They're Gone for Good.''