The sea, the sea

That's not what architects knowing something of O'Donnell + Tuomey's haunting, resonant, strangely familiar buildings, might have expected. They would be more used to something like the beautiful forms of Hudson House (Navan, 1999), fusing ruined tower with late 20th-century abstract modernism (and exquisitely anti-estate agent: at Hudson, you have to cross the courtyard to go to bed); or the asymmetric shed at Letterfrack Furniture College (1991) which echoes the mountain. So at Howth, the first impression – confident, plush, but without any clear oracular form or adjustment to its surroundings – is puzzling.

But like Diller + Scofidio's Slow House (Long Island, 1989), the formal shifts of the outside of this building are incidental. It is planned entirely from the inside. It's toned-down but wilful-looking exterior is almost an accident.

Once inside, the project starts to come into focus. It's set up to direct a convivial, open-plan lifestyle at the hypnotically lovely view out across the sea to Ireland's Eye, the scrap of island that guards Dublin Bay. O'Donnell + Tuomey was appointed after a conversation with the client, who lived in a Victorian villa next to the site but was frustrated by the closed rooms. 'Looking out to sea, with the sun on our backs, we discussed our shared preference for facing north, watching the effect of the light on the landscape,' says O'Donnell + Tuomey. It's a simple aim, but done with a sophistication that takes a long time to appreciate.

The practice's description that the house is planned 'like a telescope' only does it justice in that the shape is a function of the viewing effects inside. You can see clean through the house as it slopes down the hill to capture this view. It is set up in intersecting, curving volumes, angled to fractionally different prospects. The big living area has the austere view of island and sea, the kitchen/dining area is more cosily focused on Howth port. The spine wall separating these intersecting volumes runs diagonally between trees at opposite corners of the site: a tree frames the kitchen view with the ideal composition of a painting.

The setting out is so carefully done as to create curious effects. From inside, the house seems intensely deep plan; from the sunny back garden, this whole deep volume seems to fold up flat into nothing more than a picture frame. These optical effects are almost unnoticeable, it's your relationship to the view that seems to be changing, with the house acting invisibly as the medium. Wherever you are, the view is distinct – and seems at its optimum.

The main spaces are dull, clay-coloured, intensifying the luminous panorama, with the reveals of walls cutting across in strong colours – the client is constantly repainting, slightly varying the tone to adjust the effect. Dark wood floors or stone tiles line up with the shuttering on the exposed concrete ceiling. Brightly coloured mosaic tiles line the top-lit, concealed bathrooms; the timber-lined lightwell in the centre of the house catches the low sun and faintly glows into the living area beneath.

These complexities of the plan are well hidden. Concealed in the flank walls are the 'servant' areas; bathrooms, cupboards, stairs to the lower-ground floor with its utility room, nanny flat and garage. Within the walls of this optical device is a plan that works, sideways on, like the plan of the classic country house from Hardwick Hall on.

It's perverse that a house so intensely refined about view and perception should pay so little heed to its external form. O'Donnell + A A Tuomey explains that the outside was hardly designed at all, simply left in the shape its complex planning determined – then given a 'haircut' to level the roof, and 'smeared all over with grey pigmented limewash'. This seems partly resistance to the planners' usual preference for matching unremarkable, obtrusive neighbours and partly resistance to turning a machine for viewing into an object of attraction.

It seems the building wants to be invisible – as, from the Howth quayside, it is. (You can see its neighbours, but you can't see Howth House at all.) But close up, it isn't. And it has echoes of other forms — the Melnikov studio appears in one sketch; the client calls it a Martello Tower – as though a powerful physical presence had begun to appear but was chopped back down.

It shouldn't be imagined that this is a rejection of formality by these leading interpreters of resonant form. Their new (unfinished) gallery in Cork takes beautiful, highly expressive forms into new areas of refinement. And plenty of their former projects are far more important for what they do (the Irish Film Centre's urban function, circulation, scale, the very roughness of its finishes) – than for how they look. They are not meant to be beauty queens and the Howth House is not, the architect explains, a 'blonde'. Whatever an estate agent might think.