In Dublin, 400 officers who have training in riot situations, will be on standby for five days from Saturday, June 15, until the following Wednesday.
And a similar number of officers are expected to be deployed in Co Sligo during the two days of the actual summit, which happens on June 17 and June 18.
The PSNI has already admitted that dissidents attacks are likely to occur in Northern Ireland during the G8 summit. Assistant Chief Constable Alistair Finlay, from the Police Service of Northern Ireland (PSNI), said such attacks are part of the "normal backdrop" of life in the region.
Mr Finlay said: "I've got no reason to say that dissidents won't do something during that period of time.
"This is the normal backdrop. This has been the backdrop, the sad reality of Northern Ireland over quite a period of time."
The New IRA and ONH (Oglaigh na hEireann) are currently the most active dissident groups, he said, which have "real capacity and capability".
The operation, the biggest in PSNI's history, will see 3,600 officers drafted in from forces across the UK, 2,500 of whom specialise in public order. Another 4,400 officers from PSNI are also due to take part.
Various protests are planned around the time of the summit, with the biggest on June 17 attracting a possible 20,000 demonstrators.
A "small number" of protesters will travel from the UK and Europe.
Senior sources here explained that the officers in Sligo will be based there because of that county's close proximity to where the summit takes place in the five-star Lough Erne Resort in Co Fermanagh.
"It is only across the lake really so there will be a massive presence of public order units in Co Sligo in case there is any trouble on this side of the border.
"A lot of resources will also be put into Dublin for a five-day operation and specialist public order training of these gardai has been taking place for a number of weeks now.
"For those five days, the officers in Dublin will be based in the Phoenix Park and at other strategic locations across the capital in case there is any trouble.
"Gardai are of course always aware of the ongoing threat from dissident Republicans, but will also be monitoring a planned demonstration in Seville Place. Information has also come in that some of those groups that were involved in the May Day protests a number of years ago are also planning to protest again.
"A robust plan is in place and gardai will be ready for any eventuality," a senior source explained.
Apart from the massive deployment of riot squad gardai, the elite Emergency Response Unit will also be involved in next month's security operation. The ERU will be deployed in patrolling Border routes into Northern Ireland, including the network of waterways that could be used to launch attacks on politicians and top economists.
There is also expected to be an even greater level of co-operation between gardai and the PSNI while the summit takes place.
Eight world leaders will jet in for the summit, including US President Barack Obama, German chancellor Angela Merkel and British prime minister David Cameron.
Earlier this week, gardai were given new powers to block mobile phone coverage in the event of a severe terrorist threat.
Justice Minister Alan Shatter told the Dail that the measures were intended to deal with serious, imminent threats to life or property.
"It is regrettable that terrorist organisations and criminal gangs are prepared to exploit any means, including advances in technology, to kill and maim. It is necessary, in those circumstances, for the law enforcement authorities to be given the means to counter these threats," Mr Shatter said.