By chance, I arrived in Dublin this week on the day that the Saville report on Bloody Sunday was published.

By a weird irony, I had long ago arranged to come to Ireland to be interviewed for a television documentary on the IRA in the early 1970s, when I was cutting my teeth as a reporter on my first big story. So in the morning, I was recalling the activities of Gerry Adams and Brendan Hughes and Martin McGuinness – whom I remember in Derry in 1972 as a cold, ruthless, rather frightening figure – and in the afternoon, I was wading into the Saville report. Eloquent though it is – and wonderful in its respect and declaration of innocence for the Catholic dead – I'm not sure it's quite the Word of God that it has been made out to be. The Irish papers oozed with admiration for Saville, but largely missed the expensive soap which Saville used to clean the reputation of both the Tory government of the time and the British Army hierarchy.

Thus the commanding officer of 1st Battalion, the Parachute Regiment, got blamed but all the generals received a splendid bill of health. No conspiracies. Nobody at the top wanted Bloody Sunday to happen. But what about General Robert Ford who shouted that day in Derry "Go on Paras, go"? What exactly did he mean? And if the senior officers at Lisburn were so innocent, how come they created – and they had to be the ones to do it – the cocktail of lies that were fed to the press? Reading Saville, I realised that his report shone like gold because the Widgery report was so dishonest. Anything looks good against Widgery. It's a bit like Lord Blair, when he used to tell us "we" were better than Saddam Hussein, was that really to be the baseplate for our morality? So with Saville.

Yet looking back on those terrible years, I am horrified at not just the gullibility of us reporters but at our sheer ignorance. Like most of my other young colleagues, I had a university education but no real knowledge of Ireland. There were few good books then on modern Irish history. And I fear that many of us – despite our liberal upbringing and our acknowledgement of Stormont's injustice to the Catholics – were under the subconscious influence of darker images; the old Punch cartoon, for example, of the drunken Irishman holding a cudgel with which he would without any reason murder the refined young Englishmen who kept invading his country

We knew that Ireland was neutral in the Second World War, and heard that Taoiseach de Valera had paid a visitor's condolence to the German legation on Hitler's death and an Irishman on the west coast had refuelled German U-boats. Ireland was indeed neutral and de Valera did his make his notorious condolence visit. But the U-boat story was a lie.

Years later, studying for a doctorate in politics at Trinity College, Dublin, I spent five months travelling down the Irish west coast to investigate the U-boat claims. I read all the Irish government's coastwatching reports from 1939 to 1942. I visited old men and women in remote villages from Donegal to Cork, some of them wartime coastwatchers. In the archive at Kew I'd found records of the Tamara, a Royal Navy tugboat disguised as a trawler that went vainly hunting for U-boats along the Irish west coast.

Lieutenant Commander W R Fell of the Tamara even went ashore on Sherkin Island in Co Cork where his vessel "was boarded by the most plausible scoundrels. They begged or stole anything in reach ... one old man reputed to be worth thousands had trousers patched with paper. All would commit any crime for a shilling". I eventually tracked down villagers from Sherkin who remembered the wartime pauper – his name was Louis Nolan and he was the harbourmaster's brother – but they said it was Nolan's shirt that was patched with newspaper, not his trousers. And there were no U-boats.

So thoroughly did I check out every possible lead that I came up with three U-boat stories, at least one of which is absolutely copper-bottomed. The first was a U-35 which sailed in to Dingle Bay on 4 October 1939 to put ashore the crew of a Greek ship called Diamantes which it had sunk 40 miles west of the Skelligs. Those were the days when U-boat crews tried to be honourable to their victims, who in this case were duly taken by the police to Michael Long, the Lloyd's agent in Dingle. Local rumour, however, suggested that the U-boat captain had bid his captive goodbye with the astonishing adieu: "Give my best wishes to Micky Long." Had he been one of the many German "tourists" travelling in Ireland before the war? When my thesis was later published as a book, I received a polite letter from the long-retired U-boat captain. He had never been to Ireland before the war, he told me. And he had never heard of Michael Long.

In Kerry, a man called Michael O'Sullivan said that as a boy he had been taken by an old man with a donkey and cart loaded with a pig, cabbages and potatoes to Brandon Creek where a U-boat crew collected their supplies. O'Sullivan said the crew had ribbons down the back of their hats – this was a normal part of German naval rating's uniform – and I thought this gave credibility to his memory. There was also a fisherman in Donegal who told me that a barnacle-encrusted U-boat once surfaced beside him and that the crew asked for fish – and paid for the catch in Irish currency! But Hugh Wren, official coastwatcher between Ballybunnion and Dingle between 1939 and 1944, remarked to me, "Most of the submarines had been seen in pubs."