Swallow crossed the room to fetch the water jug that Mallon’s clerk, Jack Burton, always left, freshly filled during the afternoon. Without asking, he added an equal volume of water to each of the tumblers. It was akin to a liturgical process, repeated and perfected between them over years of working together. Mallon raised his glass.

‘Good luck.’

Swallow did likewise.

‘Good luck, chief.’

Mallon threw back half of his drink in a great swallow. He put another shot in his tumbler and did the same with Swallow’s.

‘So,’ he grinned, ‘tell me how married life is treating you.’

‘Ah, much the same as unmarried life, I suppose,’ Swallow laughed. ‘You’ve possibly guessed, Maria and I are going to be . . . blessed . . . very quickly. You know what I mean, sir.’

‘Aha,’ Mallon said. He raised his tumbler of Bushmills again. ‘Now I know I’m not losing my touch as a detective. I should have guessed that. Congratulations. When’s the big event due?’

‘I’m no expert,’ Swallow chuckled, ‘but I did my stint in medical school, as you know. It’ll be August. Early August.’

‘Then you’ll call him Augustus or Augustine, if it’s a boy,’ Mallon grinned. ‘I don’t know the female equivalent.’

‘It would be Augustina, I suppose,’ Swallow reflected. ‘Bloody awful name though.’

‘I agree,’ Mallon said gravely. ‘Forget I mentioned it.’

He threw back the rest of his Bushmills.

‘I think I’ll call it a day, chief. Thanks for the drink. I’ll be down in the morning for the reception committee. We’ll astonish Major Kelly and his amadáns with our courtesy and co-operation.’

He left the Castle via the Palace Street gate and turned in to Dame Street. A perfect half-moon had risen behind the Chapel Royal, shimmering in the clear, cold sky.

The walk to Thomas Street through the sharp air cleared his head and dispersed the effects of Mallon’s Bushmills. Two greatcoated constables watching from the shadowed corner of Meath Street nodded as they recognised him. In spite of the chill of the evening, Swallow envied them the uncomplicated simplicity of their task.

As the night went by, he knew, they would be checking and questioning those moving about on the streets. They would be noting names, times and destinations, as would scores of their colleagues, singly or in pairs across the city. Pages in notebooks would be filled with seemingly pointless detail. And yet, in there amongst the scrawls and the scribbles, there might be a clue or a connection that could solve a murder.