Television: Undercover in London, behind bars in Dublin

As an espionage thriller London Spy (BBC Two, Monday) reveals itself slowly. We get the London bit from the first scene – beautiful blue-grey shots of the city waking up, aerial angles revealing its scale and density – and there is sweet, vulnerable Danny (Ben Whishaw), messed up and lost with his dead-end job by day and drug-fuelled clubbing by night.

Danny’s chance meeting with an investment banker, a neat freak named Alex (Edward Holcroft), shapes the first of five episodes. It is for the most part a tender, almost wistful love story between two gorgeous, very different men.

The spy part of the title kicks in towards the end, although there are subtle hints throughout that Alex is not who he says he is. But then, searching for his lover, who hasn’t returned his calls, Danny finds a dead body in a secret room in Alex’s flat – in a suitcase near an electric fire to speed up decomposition, in a particularly grim touch. And a sort of truth emerges.

Nothing that Alex has told Danny is true. Questioned about Alex’s death – assuming that it is him melting away in the trunk; I think not – Danny turns to his mentor and father figure, Scottie (Jim Broadbent), for help. As well as being a little in love with Danny, the older man is a Whitehall mandarin of some sort, and Broadbent gives this supposedly avuncular character a subtle but sinister edge.

London Spy revels in its almost languid pace, a quiet drama that for a long time seems to have only two people in its world, Danny and Alex – both actors are superb – until the deadly discovery speeds up the action and introduces a new layer of complicated people and noise.

Its director, Jakob Verbruggen, also directed The Bridge and The Fall, so he has form in delivering compelling TV. But this cleverly established scenario sets a tough challenge for the drama's writer, Tom Rob Smith (the author of Child 44): he has to sell the idea that Danny, a warehouse shelf-stacker, can sharpen up those puppy-dog eyes, penetrate the world of global espionage and get to the truth about what has happened.

Everything about London Spy defines top-quality drama: the acting, production values, stunningly beautiful cinematography, and growing tension. It'll be worth hanging around to see if it works.

The voiceover in the opening scenes of The Joy (TV3, Monday) tells us, a bit unnecessarily, that Mountjoy Prison, in Dublin, is open 24/7, 365 days of the year. What? It doesn't close for Christmas? Or send the inmates on holliers in August?

That’s the thing about an observational documentary about life in such a significant institution: there is so much we already know, or at least suspect. It’s going to be noisy; young men will play up for the camera; prison is tough and boring; and the system, as represented by its correctional officers, will come off looking well.

This isn't investigative reporting, and institutions don't let cameras in for nine months unless there's something in it for them. No one could come away from the first episode of The Joy without thinking that prison officers have a tough job in a system creaking at the seams.

What this four-part series is good at capturing is the idea that this is a city within the city, its own world that replicates much of what goes on outside. There are gangs that must be kept apart, there’s violence and there are drugs – keeping drugs out of the prison is, according to one of the officers, “a cat and mouse game”.

A very different big house is the subject of Ballyfin (RTÉ One, Tuesday). This show sounds as if it will be a corporate video, but I stay until the last minute, because the story of the vast Laois estate is simply too good, with a solid list of engaging contributors who know how to tell it.

Built as a lavish and very showy family house in the early 19th century – the motto of its builders, the Coote family, was the enviably spendthrift “cost what it may” – Ballyfin was for most of the 20th century a boarding school.

The reminiscence of one of it past pupils about his loneliness at the age of 12 and the school's harsh regime is very moving. Then Ballyfin was bought by "Fred" – that's how he's referred to, first name only. He is Fred Krehbiel, a US businessman and true visionary who, disappointingly, is not interviewed in the film.

Ballyfin is regarded as the most challenging and successful restoration of a private house in Ireland. Looking at the work, which included restoring a magnificent conservatory by Richard Turner, I find it easy to believe.

Indeed, I would prefer the one-off film to stay on the meticulous restoration work – we see only glimpses of craftsmen and craftswomen – rather than to segue towards the end into a full-blown puff piece for the hotel. It would be a better and more timeless film without it.

So Downton Abbey (TV3, Wednesday) is over, at least until the Christmas edition. Given the number of loose ends in the final episode – by far the best in a characteristically corny series – it will take about five hours to sort everything out.

This week’s big set piece sees Lady Mary – statues of the Virgin Mary are more animated – getting married, in improbable, jumpy editing haste. It’s quite a reward, given that she spends the entire episode being a spectacular wagon to everyone, and spoils Lady Edith’s dreams of marriage by revealing to her intended that Marigold is in fact Edith’s daughter.

Terrific comic moments include a caper involving a mortified Mrs Patmore’s fledgling B&B getting a name of a house of ill repute and the Granthams saving the day by having tea there to restore its reputation. (Wildly improbably, I think.) My favourite moment is when Dowager Countess of Grantham’s butler, Spratt – a terrific character – is revealed as the anonymous agony aunt on Edith’s newspaper.

But there is so much left over for Christmas Day. There will almost certainly be a birth – probably Anna’s baby. There could be a marriage: surely poor Edith deserves some happiness? And what about Mrs Patmore and the pigman?

Hey, and maybe a death. It's unseasonable in terms of cheer, but Julian Fellowes, Downton's creator and writer, has done it before, with Matthew driving into that tree a couple of series back. And, after all, as Maggie Smith has pointed out in several interviews, her character must be about 110 by now.

tvreview@irishtimes.com