Sean Ryan is an agent for the British Secret Service.
He certainly did not set out to be. He was a member of an Irish gang who had come to England to make an unscheduled withdrawal from a British bank and then head home. Most made it but Ryan did not. He was sentenced to fifteen years at maximum security.
Ryan had not been a member of the IRA, despite what most of the British authorities thought, nor had the other members of the gang. The group were just criminals in need of money and the bank looked like it had some. When Ryan was caught, the others laid low for several months, expecting that he would eventually break under interrogation and give them up but Ryan was steadfast and held his tongue.
Whether Ryan was or was not an IRA member did not seem to matter to the Major, the aged department head who came to visit Ryan five years after the conviction and one-third of his way through the sentence. The Major needed a man with similar skills and background to that of Ryan. Mainly he needed a man who could hold his own in any scrape, was tough enough to win in the end, and who appeared to have a criminal background. Since using someone real was easier than making up a cover, he went to the prison holding Ryan and made an offer. Help on this one job and the remaining ten years of the sentence would be forgiven.
Ryan took the offer. Later, as the series continues, he opts for accept the offer of further employment. He knew he had a choice but when you are broke, unemployed, and an ex-con, there is not a lot of better options.
Ryan is a fighter and usually a winner. He is a hard man when he has to be but he is not cold by nature, just by profession. He likes the idea of belonging to something and the job offer that comes his way from the Major seemed like a good idea after reflection. And since he really had no political leanings against working for the Brits, he gets along well with his boss. And it is a legitimate job which for an ex-con is not easy to come by. It is not nice work. As the Major tells him, someone has to clean the streets and while it can sometimes get quite dirty, it has to be done nonetheless. Ryan sees no reason it cannot be done by him.