Dean Fitzsimons and his father Kenneth (42) were caught with loaded pistols in 2019
Dean Fitzsimons (22) was serving the sentence when officers found the phone in Wheatfield Prison, a court heard.
His mother had died after Fitzsimons went into custody, his defence said.
Judge David McHugh imposed a one-month sentence, consecutive to a sentence he is already serving.
The defendant, with a previous address at Corduff Place in Blanchardstown, admitted possession of a mobile phone in Wheatfield Prison on June 9, 2022.
Defence solicitor Simon Fleming told Blanchardstown District Court Fitzsimons was serving a lengthy sentence and was not due for release until November 2024.
Mr Fleming said Fitzsimons had co-operated with prison officials and had not caused any trouble. He also said the defendant had been punished for his behaviour in the prison.
The solicitor asked Judge McHugh to be as lenient as possible, saying Fitzsimons’ mother had died after he went into custody.
Fitzsimons, then aged 19, was jailed for seven years in 2019 for a range of offences, including possessing firearms.
His father, Kenneth Fitzsimons (42), was also jailed for possession of weapons and dangerous driving.
The father and son were caught with loaded pistols before Kenneth Fitzsimons led gardaí on a high-speed chase in west Dublin on June 23, 2019.