Neutrality, like sovereignty, is only effective if it is recognised and respected by other powers. Neither is neutrality generally quite as black and white in practice as it is in theory. During World War II, for instance, all the European states which managed to remain neutral (Ireland, Portugal, Spain, Sweden and Switzerland) treated the various belligerents differently, usually by favouring whichever was most powerful in their neighbourhood.

The Swiss and the Swedes were less rigorous towards the Germans than a strict interpretation of the rules would suggest, while Irish neutrality was interpreted in a way that benefitted the Allies. Spain under Franco was pro-Hitler, while Portugal allowed the Allies to use bases in the Azores.

Particularly during a long war, declaring neutrality is only the start of a process, a balancing act which must continue as long as hostilities last, constantly recalibrating to take account of changes in the balance of power, maintaining a military strong enough to deter invasion, and making sure no excuse is given to a belligerent to attack.

Which is where this book comes in. Ireland’s Secret War is the tale of how a succession of German spies were tracked down and caught by the Irish authorities, principally military intelligence, G2, led by Colonel Dan Bryan (who is very much the hero of this book).

The Nazi spying operation had a number of aims – gathering information about the 26 counties, including vital weather reports; spying on British and American forces in Northern Ireland; and using Ireland as a base for infiltrating Britain.

But by far the greatest threat to Irish neutrality was the German attempt to link up with the IRA, with the assistance of a motley collection of Irish people who had washed up in Berlin. These included the writer Francis Stuart, who was involved in propaganda broadcasts to Ireland, and the odious antisemite Charles Bewley who had been – to the everlasting shame of this State – our diplomatic representative in Berlin in the run-up to the Second World War.

A more serious character was Frank Ryan, the left-wing Republican captured by Franco’s forces during the Spanish Civil War, before being sprung with German help. He made an attempt to get back to Ireland by submarine with Seán Russell, the former Chief of Staff of the IRA.

Russell had travelled to Germany via America to seek Nazi assistance. Russell’s supporters indignantly deny that he was a Nazi collaborator, despite the fact that he died while on a Nazi U-boat, travelling to Ireland on Nazi orders, to do the bidding of the Nazi war machine.

One of the most senior Germans involved in dealings with Ireland was SS officer Edmund Veesenmeyer, who worked closely with Frank Ryan on plans to use the IRA against targets in Northern Ireland. Veesenmeyer, who was regarded as an expert in staging coups, later oversaw the transportation of more than 400,000 Hungarian Jews to extermination camps. It was fortunate that he never got the chance to implement Nazi policy in Ireland.

Nazi spy Herman Goetrz

It was fortunate also that Russell and Ryan never made it back to Ireland. An effective link between the Germans and the IRA would have increased the chances of invasion, either by the Germans or by the Allies trying to forestall them.

The Germans also sent a series of their own agents to Ireland, though most of them were, according to McMenamin, "little more than con artists and opportunists". Dan Bryan and his colleagues were successful in rounding them up soon after their arrival.

The one who – nearly – got away was Herman Goertz, who managed to evade capture for over a year, though his efforts to make use of the IRA failed, partly because that organisation was utterly inept, and partly because Goertz himself was somewhat unstable. Bizarrely, Bryan and G2 were aware that he was in Ireland because postal censors intercepted postcards he had sent to his family in Germany!

Bryan was very anxious to keep the British fully informed about the presence of Görtz, and the efforts to catch him, because he didn’t want London to suspect secret dealings between the Irish government and Germany.

But while Bryan was very keen on sharing information throughout the war with MI5 and the OSS (the American forerunner of the CIA), he got little or nothing in return. Some of his colleagues in G2 were less than impressed, particularly Commandant Éamon de Buitléar. At one point, de Buitléar didn’t tell Bryan that Richard Hayes, the Director of the National Library who had taken up code-breaking during the war, had broken an important German code. This information would have been extremely useful to the British code-breaking operation in Bletchley Park.

Nazi codebreaker Richard Hayes

As soon as Bryan was told about the breakthrough, he told London. As McMenamin notes: "The British were... hugely impressed with Bryan, who had, in their estimation, gone over his colleagues’ heads to help them." The rights and wrongs of Bryan’s approach deserve more dissection than they are given here – there is certainly an argument that he was too accommodating to the Allies.

The author very fairly acknowledges that this book does not claim to be a "definitive history of the war in Ireland", but is instead "intended to illuminate this period of history for the casual reader". And it succeeds in that aim – while those familiar with the story may not learn a great deal, those who don’t will find an engaging tale.

More problematic, to this reviewer at least, are the claims made in the book’s subtitle about "the lost tapes that reveal the hunt for Ireland’s Nazi spies".

The tapes in question were made by American academic Dr Carolle Carter in the early 1970s, when she recorded interviews with a number of those involved in efforts to catch German spies in neutral Ireland, principally Dan Bryan and Richard Hayes.

Dr Carter used the interviews as the basis for her book The Shamrock and the Swastika, published in 1977. So, while the tapes have not been heard for 50 years, the information they contain has been in the public domain for all that time. It is open to question how much their "rediscovery" really reveals about the subject. Nor were the tapes at any stage "lost" - they were in Dr Carter’s attic.

Those caveats aside, this is a useful addition to the literature on an important chapter in Irish history, particularly for those new to the subject.

Ireland's Secret War: Dan Bryan, G2 and the lost tapes that reveal the hunt for Ireland's Nazi spies by Marc McMenamin is Gill Books, publication date 14/4/22