The Government has agreed to the change following a recommendation from the National Immunisation Advisory Committee (Niac), according to the Irish Examiner.
The current gap is eight weeks, and it previously stood at 12 weeks.
Government made the decision so that people in their 60s will be eligible for their second jab sooner amid concerns over the surge in cases linked to the more transmissible Delta variant.
The decision is expected to be announced tomorrow when Taoiseach Micheál Martin addresses the nation on the possible changes to the reopening plan.
He has previously promised all people aged 60-69 will be fully vaccinated by 19th.
Meanwhile, Tánaiste Leo Varadkar has said a change in guidance on using the AstraZeneca and Johnson & Johnson jabs in younger age cohorts will help speed up the vaccination programme.
The Tánaiste has said Niac will consider the use of the vaccines in younger people.
The State’s Covid-19 vaccine advisory group is assessing how a possible change in advice would affect the spread of the virus.
Mr Varadkar said that Niac will make a decision and give advice based on the best medical and scientific data.
“There are some decisions that are more in the political domain than others, and I think that this issue, particularly around the use of medicines and vaccines, is very much in the domain of the experts rather than politicians,” he said on Monday.
“We are not going to be in a position to overrule their advice when it comes to something purely medical and scientific like the use of vaccines.
“We will be guided by what they say.”
Currently, the AstraZeneca and Johnson & Johnson Janssen jabs are recommended for over-50s only due to incidences of rare blood clotting being linked to the vaccines.
The Government would like to see a change in advice so that surplus AstraZeneca vaccines arriving in the next few weeks can be used on younger age groups.
Mr Varadkar said it would be “unfortunate” if hundreds of thousands of vaccines cannot be used.