The National Public Health Emergency Team (Nphet) predicted around 50-100 Covid-19 cases per day by Christmas following six weeks of Level 5 restrictions.
Government initially rejected Nphet's proposal but the restrictions will now come into effect from midnight on Wednesday.
A six-week lockdown will see numbers fall from 50-100 per day according to Nphet, while cases will not return to above 300 per day until early January.
However, failure to enforce a six-week lockdown would have resulted in the spread of the virus becoming unmanageable, according to the letter which was seen by The Irish Times.
The letter, which was sent on October 15th, expressed concern at an 82 per cent increase in cases leading up to that date compared to the previous seven days.
The Irish Times report that the letter also expressed concern "in relation to health system capacity" coming into winter, “which has traditionally been the most challenging for the system”.
“Current levels of transmission are already challenging our testing and public health capacity.”
Nphet also warned that hospitals will have difficulty in meeting demand in the next few days and weeks.
The group also warned that Level 3 restrictions in Dublin and Donegal had "no sufficient impact".
The risk is too great to wait to take further action,” the letter adds. Nphet said Level 5 was "warranted".
“Nphet firmly believes that an escalation to Level 5 is inevitable and that the earlier this proactive action is taken, the greater chance there is of quickly reversing the current trajectory."
A further 1,031 cases of Covid-19 were confirmed by the Department of Health last night.
No further deaths were announced, meaning the total number of Covid-19 related deaths in Ireland is still 1,852.
The figures brings the total number of cases in Ireland to 50,993.
