WITH the hopes of an end to one nightmare in Northern Ireland comes the dread of another. For senior police fear that the men to whom terrorism has become a way of life will switch to crime, turning Belfast into a latter-day Chicago with rival gangs battling, Capone-like, for the rich pickings of peace. Their fear is based on the fact that the young men of the paramilitary groups have grown up in a terrorist culture in which crime is used to finance political struggle. In their world it is seen as moral to hold up banks, to run protection rackets and to turn out thousands of pirated cassettes and compact discs. And the workers in these crimes-for-ideals industries are otherwise unemployed. Until now, they have received their dole from the British taxpayer and, in the case of IRA men, a top-up of about GBP60 (HK$714) a week. With the coming of peace, they will be forced back on to their basic unemployment pay. Few of them have any real qualifications. They were born into terrorism, some of them in families in which three generations have been unemployed. Through terrorism they were taken care of by whichever organisation they belonged to and achieved the same sort of ''respect'' accorded members of the mafia. All that will be taken away if the cease-fire becomes permanent. Without qualifications there will be no place for them in the modern industries which are expected to be founded with American and EC aid. They will be like demobbed servicemen in the old days, thrown on the scrapheap. In these circumstances, it is a very small step from political crime to crime for survival and then for gain. Sir Hugh Annesley, Chief Constable of the Royal Ulster Constabulary (RUC), is in no doubt about the danger. Discussing the possibility of peace in July, he said: ''I do not believe the Loyalists would stop involving themselves in crime and neither do I believe the Provisionals would stop involving themselves in crime. ''I believe the bulk of what we all define as terrorist violence will broadly stop and I think people within those groupings will turn to crime, organised crime, drugs, money laundering, intimidation and blackmail.'' It is not generally appreciated outside Northern Ireland, but, just as in gangland Chicago, the tentacles of crime extend into virtually every aspect of the province's economic and social life. There are forced collections for ''the bhoys''. Bars, clubs and restaurants pay protection money as a matter of course and write the payments off as expenses. The building industry is riddled with racketeering with VAT (value added tax) and income tax being skimmed off by subcontractors who pay a percentage to the IRA. Businessmen whose enterprises fail can always call in one of the paramilitary groups to plant a firebomb in their premises. They collect the insurance and compensation for a terrorist act and the bombers take their cut. More recently, the trade in pirated goods has flourished with cassettes turned out in small factories in the south and sportswear bearing fashionable labels imported from China. These are then sold in boot sales all over mainland Britain. The IRA has also become skilled in the use of banks, the international money market and legitimate businesses to launder its criminal gains. If it needs money in an emergency, it turns to the banks again - by mounting armed robberies. In January last year, the IRA took GBP2.7 million from the Allied Irish Bank's cash holding centre in Waterford in the Republic. The most worrying development is the involvement of the paramilitaries in the drug trade. So far, the Ulster Volunteer Force has been the main group to get into drug running while the IRA has remained aloof, even kneecapping some dealers. But there is mounting evidence that the business is now so profitable in Ireland that the IRA, faced with running costs of GBP10 million a year, had abandoned its principles and joined in. With the coming of peace, this trade and the sub-structure of criminality will remain. Somebody will run them and tap into the millions of pounds which will be poured in to restore Northern Ireland's economy, and there is no doubt that it will be the unemployed terrorists, well-armed, well-trained, and well-versed in the operations. They also have another necessary qualification: they are completely merciless. An indication of what might be in store came with the IRA's killing two weeks ago of Martin Cahill, Ireland's top gangster. Cahill, known as ''The General'', worked the fringes between terrorism and crime. It appears he stepped over the mark and the IRA ''rubbed him out'' in a gangland-style killing, pumping bullets into his body. What can be done about it? If the peace holds the British army will be withdrawn and the massive intelligence organisation headed by MI5 will be wound down. This will provide its own internal problems. The withdrawn battalions might find themselves candidates for disbandment or assigned to international peacekeeping duties while MI5, which fought for the Northern Ireland role to compensate for its loss of business after the collapse of Communism, will once again find itself short of clients. More importantly for Northern Ireland, their withdrawal will remove important forces from the battle against crime, which, as we have seen, is inextricably involved with terror. The burden of the battle will then be borne by the RUC which, despite setting up C13, the Anti-Racketeering Squad, is still largely aimed at countering terrorism. The irony of the situation is that Gerry Adams is already calling for the RUC to be disarmed. It may be that when the criminal gangs are shooting it out on the streets of Belfast and Londonderry he will look to the RUC for protection.