The presence of the LE Samuel Beckett at the the world's biggest arms fair has sparked outrage

The 90m-long offshore patrol vessel is fitted with a 76mm gun, two 20mm cannons and four general purpose machine guns.

It was commissioned in May 2014 and cost €71m.

It has been deployed on humanitarian missions in the Mediterranean and its crew are credited with rescuing 2,818 people.

Ship manufacturers Babcock asked for it to be sent to the multibillion showcase of military hardware and software, which is attended by 1,600 exhibitors from 54 countries.

"The attendance at the exhibition is an opportunity to portray the considerable level of commitment and investment made in recent years to a wide audience towards protecting and safeguarding Irish territorial waters, by showcasing the success of the offshore patrol vessel design and build capabilities," the Department of Defence said.

The ship's appearance on the Thames was flagged by Danish company Systematic Maritime. Some of its SitaWare command and control technology is fitted to the vessel.

The Defence Forces is the first military to use the company's software to create an operational picture for its forces over land, sea and air.

SitaWare is described as a unique command and control system used by more than 20 countries to give "warfighters at all tactical levels" better situational awareness and greater operational capability.

Edward Horgan, of the Irish Anti-War Movement and a United Nations elections' inspector, said an Irish Naval Service vessel's presence in the world's biggest arms fair was a misuse.

"It is completely inappropriate for the Irish Defence Forces to be involved in any way in this arms fair, or in the international arms market that is notorious for corruption," he said.

"And if the LE Samuel Beckett is attending this arms fair due to some contractual clause in its original purchase agreement, this raises more concerns about the appropriateness of our defence purchasing procedures.

"With more than 8,000 Irish citizens homeless, we cannot afford to waste precious resources supporting the corrupt international arms industries."

The LE Samuel Beckett is open to visitors to the naval zone at the arms fair.

Mr Horgan called on Taoiseach Leo Varadkar to explain who authorised the ship's deployment to the arms fair and whether a fee was paid for it to attend.

The riverside section includes waterborne demonstrations and interactive presentations on eight warships, including a demonstration of the SitaWare Maritime solution on the LE Samuel Beckett.

Shipbuilder Babcock, which built the vessel at its north Devon yard in England, declined to discuss whether the LE Samuel Beckett's presence at the arms fair had been written into the manufacturing contract.

"We have an ongoing relationship with Ireland's Department of Defence and the Irish Naval Service due to our successful work on the Irish OPV (offshore patrol vessel) programme," the company said.

Babcock built three Irish Naval Service ships in recent years and is in the process of completing a fourth.