Her transition into the main prison system will be gradual as the prison authorities balance her health requirement with her security needs.
Within the medical unit she will be under strict observation initially to ensure that she is coping psychologically with her strange environment. She will have had her breakfast and lunch today in this area.
However, sources said that she will most likely move to the Laurel House unit of the jail.
The units in the women's prison can cater for varying numbers with a communal dining /living area and individual rooms rather than cells.
She can then choose to mix with the other inmates or stay in her own room.
Many of her high profile predecessors like Catherine Nevin and 'Lying Eyes' Sharon Collins managed to make the transition from being a loner to mingling with a select few.
Prison Governor Mary O'Connor has the task of monitoring the mood of fellow inmates before making decisions as to her future.
"The prison first has to meet her medical needs as they were outlined by the court in sentencing and this is not a problem. But security is a major headache and one that may require a certain degree of isolation for the former judge," said a senior officer.
The Dochas is supposed to have a capacity for 105 but this is usually closer to 150, which does not take into account the number on temporary release.
The regime is relatively relaxed but the overcrowding and doubling up with bunk beds has increased tensions.
There are frequent fights, and drugs and drink find their way inside as do mobile phones.
Ironically, Perrin was a "visiting" judge in the Dublin District Court when Catherine Nevin successfully contested a charge of possessing a mobile phone.
Visiting times are flexible in the medical unit but normal visiting times are from 10am to 12 noon and 2pm to 4pm from Monday to Saturday.
Sentenced offenders like Perrin are entitled to one 30-minute visit per week. Special visits of 15 minutes may also be available on request.
Each prisoner can nominate six adults who will visit. There is no limit on the number of children.
The Dochas is far from a luxury prison, according to an independent watchdog group.
In a recent report the Visiting Committee made a scathing attack on the changing face of the jail, once regarded as a model for the future.
The Committee hit out at a serious violation of the human rights of a prisoner and other "wholly unacceptable practices".
The body stated bluntly in its 2010 annual report: "It is with regret that this committee notes a distinct shift from the ethos on which the success of Dochas was built and the committee express concerns over future direction of the prison."
Unacceptable situations which arose included the censorship of incoming items resulting in women not having access to fresh underwear on occasion in 2010 and a lack of toothbrushes on another.
Of "deep concern" was a search carried out by a dedicated search team using a BOSS chair which can detect drugs concealed in the body.
Women were required to remove clothing including underwear in the presence of male officers. A small number of towels were passed among the women, a number of whom were not adequately covered.
The Committee held that when using the BOSS chair it is unnecessary to subject women to the humiliation of stripping naked and is not in keeping with international norms of good practice.
A death by suicide also took place in August 2012. Ruth Murphy (48) was serving a life sentence for the murder of her seven-year-old son Karl.