From 7am until shortly after 3pm, specialist prison officers who were later backed up by gardai, conducted the operation at maximum security Portlaoise Prison.
Sources said that information on the seized phones and the computer equipment will allow gardai to gather intelligence on hundreds of criminals connected to the Real IRA and other dissident factions.
The huge operation has also led to a deeper realisation within the ranks of dissident republicanism that detectives have cultivated a large number of informers.
"These seizures have absolutely massive implications - the information that will be found on those phones should point to assassinations, terrorist and extortion plots and simply who these individuals were communicating with on the outside," a senior source told the Herald.
"Because so much was got, it will be weeks before the full implications of what was discovered will be realised."
The raids happened in the notorious 'E' block of the jail, which has three landings and houses just 48 prisoners from different dissident republican groups, including some men considered among the most dangerous terrorists in the State.
A large number of the phones were found in the recreational area that the dissident prisoners use, while others were found in cells and others on searched prisoners.
Dissident sources have told the Herald that the "widespread belief" among their ranks is that the raids are a result of information provided by a man who is due to be a State witness in an upcoming murder trial but gardai would not comment.
A Prison Service spokesman confirmed that the searches had started at 7am and were "intelligence led".
It was the biggest operation at the prison since slain Finglas criminal John Daly made an infamous phone call to the RTE Liveline show in April, 2007, from his jail cell.
Two budgies, 17 mobile phones, three sim cards, 11 chargers, eight batteries, 150 ecstasy tablets, 30 syringes and dozens of containers of home-made brew were seized in raids targeting gangland elements in the jail after Daly's call.