It made up 55% of all cases last week.
The Delta variant of Covid-19 will 'almost certainly' be dominant here in two weeks' time, according to the director of the National Virus Reference Laboratory.
Up to 55 per cent of cases last week involved this strain.
Eoghan Murphy reports:
"Dr Cillian De Gascun says the Delta variant, first discovered in India, is at least twice as transmissible as the Covid strain we experienced last summer.
He says there has been a 'dramatic increase' in cases of it here over the past two weeks.
It made up less than 9 per cent of cases between June 7th and 13th - but Dr Gascun says this rose to up to 55 per cent last week.
This will be confirmed by further analysis in laboratories, he says.
Dr De Gascun says this will almost certainly lead to 'Delta dominance' by the middle of July - leading to an increase in case numbers, hospitalisations and deaths in the following weeks.
He says the Delta variant is already dominant in the UK, accounting for about 97 per cent of cases - with case numbers, hospitalisations and deaths increasing in recent weeks. "