Sir, – Congratulations to Dublin City Council and its chief executive Owen Keegan for getting to grips with the problem of derelict sites in the centre of Dublin ("Dublin City Council to crack down on owners of derelict sites", December 29th). But he suggests that we have been "not cognisant of the negative externalities". What on earth are those? – Yours, etc,
Dr PETER BOYLE,
Blessington, Co Wicklow.