Security is being increased along the Shannon/Erne waterway to prevent the river being used as a drug-running route or for the transportation of illegal immigrants in or out of the UK.
The potential for using the rivers as a corridor for the movement of both drugs and illegal aliens has increased since the Northern ceasefire came into effect.
There has been a cutback in security on the northern side of the Border since the ceasefire, and there have been only limited RUC river and helicopter patrols on the upper reaches of Lough Erne.
The RUC has been aware of the potential of the river as a corridor for the transportation of arms and explosives to the North, and has been monitoring river traffic since the reopening of the £30 million cross-border canal.
There has been a large number of drug seizures off the south coast in recent years from ships dumping drugs on the seabed for collection later or from ships using southern ports.
A number of these seizures took place in the Shannon estuary at the deepwater port of Foynes. Some of the drugs seized were destined for the Irish market, but most were bound for mainland Europe or the UK.
"While the criminals would have moved some of the drugs by land to the North and into Britain, that was quite a risky operation because of Border security," said a Customs and Excise source yesterday.
"We began looking at their other options for moving drugs north and the potential of the Shannon with thousands of pleasure craft moving freely across the Border from April to October," he said.
"While a river trip from the lower reaches of the Shannon to Northern Ireland would be slow, it would be relatively safe and large quantities of materials, or indeed people, could be carried on a large pleasure craft. "Part of our difficulty is that illegal cargoes could be loaded or unloaded at any point along the Shannon and it is very difficult to monitor.
"However, we are aware of the what could be happening and we have been monitoring the river traffic since the summer and this will continue."
Discreet checks have been carried out by customs officers on holiday cruise company customer lists. This was confirmed by one Shannon-side pleasure cruiser operator yesterday.
In recent years it has become mandatory to register all craft using the Shannon, and customs staff have access to these lists of privately owned boats.
Midland newspapers have quoted Garda sources confirming increased surveillance of river traffic in the fight against drugs.