A Real IRA leader who plotted an explosion during the State visit of Britain's Prince Charles in 2015 has died in prison.

Seamus McGrane, 64, died from a suspected heart attack while serving an 11-and-a-half years prison sentence for directing terrorism.

McGrane, who was also convicted of IRA membership in 2017, was only the second person to be convicted of directing terrorism in the State - his ally Michael Mc Kevitt was jailed for 20 years in 2003 for directing terrorism.

During his trial at the Special Criminal Court in 2017, the court was told that McGrane discussed an operation involving explosives in the run-up to Prince Charles' visit two years ago.

McGrane, last of Little Road, Dromiskin, Co Louth, was convicted of directing the activities of an unlawful organisation, styling itself the Irish Republican Army, otherwise Oglaigh na hÉireann, otherwise the IRA, between the dates of 19 April and 13 May 2015.

He was also convicted of membership of the IRA between 18 January 2010 and 13 May 2015.

Sentencing McGrane, presiding judge Ms Justice Isobel Kennedy had said that it was "a most serious offence".

The judge also noted that the court had received a letter from Fianna Fáil TD Éamon Ó Cuív, in which he expressed the opinion that McGrane was "fully supportive" of Mr Ó Cuív's efforts to facilitate the peace process.

The judge said, however, that the TD's opinions were "unconvincing" in the light of McGrane's history. 

The court heard evidence from two audio recordings, from April and May 2015, of McGrane and Donal Ó Coisdealbha in conversation in the snug of The Coachman's Inn on the Airport Road that had been bugged by garda detectives.

McGrane had issued instructions to Ó Coisdealbha regarding meeting other people and had made statements about providing bomb-making material for others.

McGrane mentioned experimenting with the development of explosives and discussed strategy and his involvement in training people in the IRA and "swearing in" people to the organisation.

The bugging also referred to a "military operation" of significance  and "the main attack" on 19 May 2015, the date that Prince Charles was due to carry out a State visit.

McGrane had also referred in the recordings to an attack on Palace Barracks, the MI5 Headquarters in Northern Ireland on 12 April 2010 and to a bomb on a railway line.

McGrane instructed Ó Coisdealbha that the operation should not be an "embarrassment" and that it was not to occur in Sligo or Galway, where Prince Charles was due to visit.

The target of the attack, the trial was told, was to be the Cross of Sacrifice, a monument in Glasnevin cemetery in Dublin commemorating British and Irish soldiers who fought in World War 1. 

McGrane was arrested six days before the planned attack.

Searches related to the plot were then conducted at McGrane's home in Dromiskin and an adjoining property at the back of his house, as well as a house at Harbour Court in Courtown, Co Wexford and a locker at Maynooth University.

The court heard that a significant amount of explosive substances were found at McGrane's house. There were detonators, glucose, ammunition and mortars. 

In Harbour Court detectives found a water butt containing rockets and semtex.

At Maynooth University, a time and power unit (TPU) and a broken circuit board were found in a specific locker which only Ó Coisdealbha, the man McGrane had been instructing, had access to.

McGrane had two previous convictions. The first was for IRA membership and dates back to 1976 for which he spent one year in custody.

The second conviction, from 2001, related to training others in the use of firearms for which he was jailed for four years by the Special Criminal Court. In October 1999 McGrane had been arrested in Co Meath in a training camp discovered in an underground bunker, where a firing-range had been constructed.

In November 2016, Ó Coisdealbha, 26, of Abbeyfield, Killester in Dublin, pleaded guilty to membership of the IRA and was jailed for five-and-a-half years.