The apartment was in a north Dublin suburb that was a throwback to the Celtic Tiger days, built close to the airport with restaurants, pubs, shops, a creche and a gym.
In Kelly's fanatical world, it was a place filled with "infidels" and "kafir". His neighbours could never have guessed that the Irish man who dressed in Arabic clothes was a jihadi in their midst, or that his portly Jordanian flatmate was his mentor.
By then, Kelly - a Christian Brothers-educated former altar boy from Dublin's Liberties who converted to Islam after a spell in a Saudi jail - was on the final stretch of his journey towards fighting for Islamic State of Iraq and the Levant (Isil).
After years of spouting fanatical extremism, defending terrorist attacks on the West, calling for Barack Obama's death, radicalising on social media and preaching from a soapbox outside the GPO, Kelly had turned from propagandist to activist.
He needed contacts in Isil, he needed travel documents and he needed money. According to security sources, the go-to person was the Jordanian, who has long been the Garda's top Isil suspect in Ireland. He was Isil's suspected fixer, who organised funds and travel documents to get people out of Ireland to the Isil battlegrounds of Syria, Iraq and Afghanistan.
In preparation for jihad, Kelly is believed to have moved in with the Jordanian, who cannot be named for legal reasons, earlier this year. Their movements were monitored by the Garda's Counter Terrorism International unit (CTI).
For several months they lived low-key lives in north Dublin as single men, although both were married with children. Kelly's wife and children lived in the UK. The Jordanian's family, including the Irish-born son that gave him residency rights here, were back in the Middle East.
Kelly and the Jordanian lived on social welfare handouts - the Jordanian paid the rent on the apartment with the help of rent allowance, according to an informed source.
They lived "a very frugal life". Rather than attending larger mosques, where their extreme views were shunned in the wider community, they attended Friday prayers with a small gathering which included other like-minded Muslims, according to a source.
"They were supported by some people in the community. These people would be well thought of. There were around half-a-dozen people in their group, and he [Kelly] was the only Irish-born person," said a security source.
The CTI raided the apartment earlier this year, while Kelly was a guest, but nothing of an incriminating nature was found.
The men were from entirely different backgrounds. After being radicalised in a Saudi jail, where he served eight months for distilling alcohol, Kelly went to London in 2002 and hooked up with the now jailed extremist Anjem Choudary, and reportedly trained with the Taliban in Pakistan.
In 2010, he returned to Ireland, and became Islamism's most vocal proselytiser, on social media and in interviews. On his LinkedIn page, he described himself as "dawah coordinator [which means to preach Islam] at islam4Ireland" and moved between rented houses in Cavan, Longford, Meath and Dublin.
He was twice arrested, before US president Barack Obama's visit in 2011 and again last year ahead of the visit of Prince Charles. He was never charged with any crime. But the Garda's principle suspicion was that Kelly was trying to radicalise young Muslims online.
"Particularly, he was suspected of trying to radicalise those who had converted to Islam. He was looking for people who he could pull into that kind of world," said one source.
Kelly was well known within the Muslim community and was generally regarded as a madman, said one Muslim man who knew him. One of Kelly's "converts" was said to be a man known to have emotional difficulties, who later had the wisdom to drop Kelly.
A handful took him seriously, he said, such as one man who befriended Kelly and began circulating Isil symbols and who now finds his Irish visa under review.
The Jordanian moved to Ireland with his family in 2000. He applied for asylum but abandoned that after his son was born here. Gardai had been monitoring him since 2011, the year of the Arab Spring uprisings across the Middle East, when he was suspected of helping people to travel.
Based on intelligence they gathered over the next four years, gardai later described him in court as a "recruiter" of Isil terrorists, a "foremost organiser" and "facilitator" of Isil in Ireland.
As with Kelly, gardai had no evidence to charge him with a crime.
Last year, the Garda moved against the Jordanian. In March last year, he was told he was being deported. The Jordanian resisted, claiming he would be tortured if he was sent back.
He reinstated his claim for asylum and launched a legal challenge, during which he vehemently denied the alleged Isil credentials attributed to him by gardai. Khalid Kelly showed up in court to support him.
On July 6, the Jordanian, having exhausted his legal options all the way to the European Court of Human Rights, was put on a chartered plane to Amman in Jordan, where he was last reported to be in prison.
Kelly had by then already left the country, making his way to the UK, and from there to Turkey - gateway to the Isil conflict zones.
On November 4, Kelly, who now went by the name, Abu Usama Irlandi, was one of four suicide bombers who drove a truck packed with explosives towards an Iraqi army base near Mosul.
He was shot down before he reached his target. Isil announced the death of Abu Usama Irlandi on its propaganda website alongside a photo of him moments before he set off on his suicide mission.
His death has yet to be verified by the Department of Foreign Affairs and the Garda.
Kelly and the Jordanian facilitator were among the two top-ranking Isil suspects in Ireland, according to security sources.
"If you wanted to travel, the guy who was deported would have been the man to go to. The Jordanian is the person that was believed to be the main facilitator," said a security source.
Khalid Kelly was the most significant Irish-born "person of interest", he said.
"Kelly was his very close associate. It was always felt that he was going to travel. He was very vocal and some people thought it was bravado. But he had strong beliefs. He had good contacts. It was hard to see how he would not have been going to travel."
Gardai could not have stopped him, without specific intelligence on what he planned to do, as they do not have powers of preventative detention.
Following years of ambiguity on the subject, Kelly's fate would appear to be a fairly stark confirmation of the phenomenon of the radicalisation of Muslims in Ireland.
Yet leaders of the 60,000-strong Muslim community here are divided, some imams warning that Islamic extremism has already taken hold while others claiming it never will.
These differences in part reflect complicated and deep divisions within Islam itself.
The dominant Sunni mosque in Ireland, the Islamic Cultural Centre in Clonskeagh - which has been linked to the Islamist Muslim Brotherhood - has condemned Isil violence. Its spokesperson, Dr Ali Selim, insists that Irish Muslims are too integrated for radicalisation to take root here. Speaking on RTE Radio last week, Dr Selim pointed out that Khalid Kelly was radicalised in London, not in Ireland.
Shayke Dr Umar Al-Qadri, on the other hand, has claimed that Irish Muslim children were being taught "hatred of other communities" in certain mosques and called for louder condemnation of extremism from Muslim leaders. Around 200 people attended his Muslim march against Isil in Dublin last year - the Clonskeagh mosque did not support it because protests are not its way of doing things.
The imam of the Shia mosque, Ali Al-Saleh, is another vocal opponent of extremism, as is Ibrahim Noonan of the Ahmadiyya community. He claimed last week that delegations of scholars and imams from academies in Islamabad and elsewhere in Pakistan, have been conducting low-key speaking tours in certain Irish mosques in recent months.
The official Department of Justice position is that extremism is here.
Minister for Justice Frances Fitzgerald told a conference in Dublin in June that online radicalisation was a problem but little was known about how to stop it. "Extremism is no longer an abstract problem that happens somewhere else. It's here," she said.
Lucinda Creighton, the former Fine Gael minister and founder of Renua, is now monitoring Islamic extremism in Ireland as a senior consultant with the Counter Extremism Project in the US. She cites several Islamic leaders who claim there are more than 100 extremists here, and evidence that Isil supporters are "using the country as a logistics hub for the movement of foreign fighters and for fundraising".
As if to reflect the new reality, the threat of an attack here was upgraded last year from low to moderate - which means possible but unlikely. The upgrade coincided with Ireland's inclusion in an Isil propaganda video listing 60 nations it claimed were in a "coalition of devils".
The big concern for security forces in Ireland, as with other European countries, is the prospect of returning jihadis - their extremism entrenched by combat and with the potential capability to mount attacks on home ground.
According to Dr Usama Hasan, head of Islamic studies at the Quilliam Foundation, an anti-extremism think-tank in London, Isil's forced retreat in Iraq may cause a "dangerous" change in strategy.
Read more: Gardai suspected 'Khalid' Kelly of plot to assassinate Prince Charles
"What's more dangerous now is that Isil has already begun to encourage their supporters not to come to Syria or Iraq but to attack westerners where they are... That is the big threat now for western governments, as Isil is pushed out of Iraq," he said.
There are already "hundreds" of former Isil fighters who have come back to the UK from Syria, he said, and "there could possibly be dozens who have come back to Ireland".
The Department of Justice says between 25 to 30 Irish citizens travelled to Middle Eastern conflict zones since the Arab Spring in 2011. But how many were fighting with Al Qaeda or Isil is unknown - many travelled to join the popular uprisings that swept through the Middle East. A reported Isil defector claimed in an interview last year that 40 Irish or "Irlandi" were fighting in Syria and Iraq.
Usama Hasan, who was himself a jihadi during the Soviet-Afghan war in the 1990s and now campaigns against extremism, says jihadis do not operate alone but as part of a cell.
"There are very few lone activists. Kelly would not have been alone. He would have had a small number of people around him, maybe five or six, maybe 50 to 60, who knows. Some would be active supporters, going to weekly meetings, and others who don't get actively involved but who are sympathetic to the cause," he said.
With Kelly and the Jordanian suspect no longer in the picture, Garda are monitoring the next layer of suspected Isil sympathisers.