The Irish Navy had to cancel patrols almost every second day last month as more staff members quit the service.
It cancelled 12 patrols from January 1 to January 26, blaming staff shortages and the bad weather.
But it now only has four ships to patrol the biggest sea area in the EU after two more vessels, the LE Niamh and the LE Roisin were taken out of service and put into storage, one of them for a refit. It had nine ships 18 months ago.
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The number of staff in the Navy has also dropped by 294 members from 1,094 down to 800. More sailors are leaving every year than joining, blaming poor pay and too many long hours at sea.
Last night, Sinn Fein Defence spokesman John Brady TD claimed the country's national security is now at risk because of the naval crisis and the defence forces are unable to police Ireland's 200 mile exclusive economic zone in the Atlantic.
He said: "In less than four weeks in January the naval service was forced to cancel 12 scheduled patrols. The staff is down to 800, 294 members below what they need.
"The Navy has been left with just four vessels to patrol one of the largest maritime areas in Europe.
"We are rapidly approaching the point of no return, a tipping point from which the Permanent Defence Forces can not recover.
"The understaffing is placing incredible strains on remaining members. One member of the naval service has reported that in one year alone they spent 320 days onboard a ship."
He accused the Government of failing to implement the recommendations of the recently published Report by the Commission into the Future of the Defence Forces.
He said Ministers had the report for six months but had done little or nothing but kick the can down the road.
New Defence Minister Micheal Martin confirmed the cancellation of the 12 patrol days.
He said: "A range of factors including adverse weather conditions and unscheduled maintenance as well as staffing issues can impact scheduled patrol days.
"12 patrol days were cancelled between January 1 to January 26, 2023.
"I acknowledge the current staffing difficulties in the Naval Service are very serious. I have received a briefing from the military management and met them on their proposed recruitment strategy for 2023. Recruitment was notably slower last year than in previous years.
"The briefing outlined how the Defence Forces' military management proposed to counter this trend."
Mr Martin said a decision was taken that the LE Roisin should be placed into operational reserve with effect from January 31 as should the LE Niamh once its midlife refit is completed later this year.
The Navy has advised this action will not affect its ability to fulfill its current maritime security and defence commitments. A new comprehensive naval regeneration plan is being progressed.
He added: "The situation is not good now in terms of the naval service. The numbers are simply too low. We need a radical look at the whole organisation in terms of recruitment and retention. We need to stand back and do everything we possibly can to arrest this decline."
The Navy's main role is to carry out a fishery protection service for the EU and to patrol all Irish water from the shoreline to the outer limits of the Irish Exclusive Economic Zone.
They also help hunt down drug traffickers and are supposed to protect the underwater internet cables connecting Ireland and Europe with the rest of the world.
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