From as long ago as I can remember I have been fascinated by spies. Had I been born in a different country I might even have contemplated joining a spy service but the Ireland in which I grew up did not then boast of such. So I remained a close observer of the espionage world and became an entrepreneur instead, safer if not so exciting.

Spying is sometimes described as being the second oldest profession (prostitution being the first). Either claim may or may not be true, certainly the desire to know – usually in order to forestall – the intentions of one's enemies is a deeply embedded desire among most rulers and it has led to the creation of what is now a multi-billion dollar global activity impacting on the lives of millions.

And the fascination that I (and much of the public) has with the profession is easy to understand. Spying contains all the elements of drama. It involves deception, dissembling, deceit, danger and occasional dollops of sex though practitioners will tend to play down that last aspect.

The element of espionage which most excites my passion is human spying (so-called HUMINT). This can be summarised as the recruitment by one country's spy service of citizens of another country who are prepared to provide secret information to that spy service in return for reward.

Okay, it's a bit more complicated than that; for a start the motivation of agents can be complex; some indeed betray for money, others for excitement, for revenge, to feel important or because they are forced to. As a former British SIS (MI6) officer told me once; "My job was to point out to the potential source the benefits of cooperating with us which includes avoiding the consequences of them not cooperating".

And it can be about more than simply providing information. An agent (agencies prefer to use the word 'source') may choose to collaborate with the intelligence service of a foreign power for a variety of reasons. They may believe that the collaboration will actually advance their own country's interests or more usually the sectional interests of the group of which they are members or affiliates. They may be quite selective in deciding what information they will turn-over and what will remain undisclosed. For that reason one of the more important classifications that spy agencies use to categorise their sources is the degree to which the agency controls their actions. "Fully under our control"; "partially under our control"; "not under our control" etc. The category of being "fully under our control" by far the most preferable.

When I started taking an interest in espionage in the mid-1960s the world was a very different place. The Second World War had been over for barely twenty years and memories of it still dominated the lives of most adults, certainly those of our political leaders and administrators. The Cold War was at its iciest and the prospect of nuclear annihilation was frighteningly real. The Cuban missile crisis of 1961 had scared the living daylights out of people. I recall being told by my father (a civil servant in our Defence Department) how, at the height of the stand-off, the US ambassador had called on our then Taoiseach (prime minister) and informed him of planned US actions in the event of the crisis escalating to open hostilities. It is likely (though never publicly confirmed) that among the actions planned would have been the seizure by US troops of Shannon Airport in the West of Ireland, then (and since) a vital staging post for moving US forces from the continental United States to other war theatres.

Technology was beginning to make an increasing impact on intelligence gathering. The sharp end of the Cuban crisis was an example of technology-led espionage (aerial photography mainly) with the role of agents on the ground in Cuba a minor one. Of course the contribution made by the GRU (Soviet military intelligence) officer Colonel Oleg PENKOVSKY was significant, he provided substantial amounts of intelligence on the Soviet Union's missile capabilities, though the leadership of the US was unsure of exactly how much credence could be placed on it, its code name "CHICKADEE" not exactly helping.

But while technical intelligence has increased exponentially to the point where it is almost all-seeing, agent-led intelligence has remained. Part of this may be bureaucratic inertia – "we're here to capture human-sourced intelligence and we're not going anywhere" – the main reason for its survival is that it still contributes real value. One example is the former NSA techie Edward SNOWDEN. While Snowden may technically not have been a spy the information trove he revealed shows just how much damage can be done by a human source on the inside who is prepared to reveal what they know. Had he been an agent of the Russian or Chinese intelligence services his value to them would have been incalculable. Indeed knowing how intelligence services operate it is difficult to see how Snowden will be able to resist the pressure from Russian authorities to 'pay' for his residency by way of passing over secret information to his hosts. He may not have started out intending to spy but it is likely indeed that he has become one by default.

It's a tough business, espionage.

One of the advantages of running my own business for all those years was the way I could use my business-related travel (to Europe, Africa, the Americas, and Asia and behind the Iron Curtain) to meet intelligence practitioners whenever the opportunity offered.

This wasn't easy, intelligence officers operate in the shadows and do not disclose their identities to those who are simply curious. But with perseverance I managed to begin the process and over time struck up acquaintances with officers from most of the major services. For example, the photograph below was taken of me in 1991 outside the then headquarters of the KGB in the Lubyanka in Moscow. The purpose of that visit was to make contact with the just-dismissed KGB General, Oleg Kalugin. Kalugin, who at the time of his appointment was the youngest ever to be made a major-general in the KGB, probably 'flew too close to the sun'. His bosses first became envious, then annoyed and then suspicious of him and forced his early retirement. Their suspicions – probably unjustified at the time – were to be later confirmed post-hoc when he effectively became a defector to the US government. This came about when he gave evidence for the US in the criminal trial of a Soviet agent in the US military reserve, Col George Trofimoff. Kalugin had run Trofimoff while he was a KGB officer. In fairness to Kalugin he came under considerable pressure from the US to testify, being threatened with deportation if he failed to do. Interestingly, the CIA never fully trusted Kalugin and barred him from entry to any of its facilities.