The telephone call came out of the blue. Ronnie Tallon was one of the country's most distinguished architects and the firm he co-owned, Scott Tallon Walker, had designed several of the country's most famous modernist buildings, including RTÉ's headquarters at Montrose.
But this was a commission like nothing else. The caller was from the Archbishop of Dublin's office and the brief was to design the "physical arrangements" - as it was quaintly put - for the impending visit of Pope John Paul II.
The call came in on July 29, 1979, just a week after it had been first announced that the Polish pontiff would be paying an official visit to Ireland. Tallon would have just eight weeks to devise the construction of facilities for more than a million people on a large open site of the Phoenix Park known as the Fifteen Acres. As well as an altar, backdrop and seating for thousands of clergy, he would have to think of water supplies, drains, electricity and telephone lines.
Today, massive events involving large crowds are the preserve of logistics and event-planners such as Arcana - the firm behind the plans for the visit of Pope Francis to Ireland - but back in 1979, such a specialist industry was in its infancy, and it fell on Scott Tallon Walker to think completely outside its comfort zone and even devise the corrals where crowds would gather and the entrance routes with which they would descend on the park.
And Tallon had to start immediately. From that phone call onwards, the architects' office in Merrion Square, Dublin 2, was a hive of activity. Hundreds of staff from construction firm John Sisk & Co were recalled from their holidays to start immediate work on the Phoenix Park site, beginning with a car park.
Tallon, who died in 2014 aged 87, succeeded in his mission. In fact, his efforts were deemed so successful that he was regularly called on outside Ireland to advise on the arrangements for huge papal visits elsewhere. And, in honour of his achievements, the Vatican awarded him a papal knighthood. He had managed to create what he later called "an outdoor cathedral" to accommodate 1.2 million people.
Next weekend will see Pope Francis say mass at the very spot where Tallon placed the altar and while the stage design will be very different, one feature created by the architect will be visible to the half-a-million people expected to make the pilgrimage to the Phoenix Park: the gleaming, white-painted Papal Cross.
This 35m structure has been a part of the everyday life of the park since September 1979 and it's become a Dublin landmark every bit as well known as the Wellington Monument, that much-earlier Phoenix Park attraction, which was completed in 1861.
The cross would prove to be the most challenging of all the aspects of Tallon's plans. The idea was to create something that would be a visual pointer to everyone who attended the monster mass no matter how far away they were from his Holiness, and it needed to be tall to create an impression.
As Tallon explained later: "We required a cross the height of Nelson's Pillar, which was 125 feet high, which would be clearly visible to all from the furthest reaches of the vast congregation and which would give a sense of focus to the occasion."
The scale of Nelson's Pillar would still have been a vivid memory for many some 13 years after it was blown up, but in the end, the cross would have to be 10 feet shorter simply because the tallest tower crane then available in Ireland could not have accommodated a larger structure.
It was built in conjunction with engineers Ove Arup & Partners and constructed over six weeks in Inchicore. The 31-stone structure, requiring six steel joists and some 5km of welding was built by specialist workers around the clock.
The time frame to design, construct and erect the cross was far shorter than a similar job would have required. As Tallon later explained: "Normally if you were doing a steel contract, it would take six weeks to get the material in and another 12 weeks on top of that to get it fabricated and erected."
But once it was manufactured, two significant problems awaited - how to get the cross from the short distance - as the crow flies - from Inchicore to the Phoenix Park, and how to erect it.
The structure was so large it was deemed impossible to take it along the South Circular Road as had first been envisaged and in the end the cross had to be transported, slowly, in the early hours of the morning as far as the city centre, across the O'Connell Bridge where the turning circle was wide enough and then up the northside Liffey quays before entering the park at the Parkgate Street entrance and along its central thoroughfare, Chesterfield Avenue.
No planning permission
It then took several attempts, using two 100-tonne cranes to position the cross into its supporting concrete chamber, but it was finally achieved on September 14 - two weeks prior to the Pope's arrival. But while the simple steel cross, visible from as far away as the Dublin Mountains, is a source of pride and for many, Catholics and non-religious alike, some have been irritated that it remains in place permanently - especially as planning permission was never secured for it.
Under the planning laws, permission was not required as it would be a temporary structure. And when Tallon was designing the cross and the other "physical arrangements", the expectation was that everything would be removed after the papal visit and that part of the Phoenix Park, next to the US ambassador's residence, would be returned to the way it had been before.
After all, there had been several precedents for having ceremonial structures removed, such as the triumphal arches erected at Leeson Street Bridge, Dublin for the official visits of Queen Victoria and King Edward VII in 1900 and 1903 respectively.
And all remnants of the huge Eucharistic Congress events of 1932 have long vanished. There was a huge mass at the Phoenix Park during the five-day event and a lavish altar and backdrop was built on the Fifteen Acres close to what's now St Mary's Hospital. No trace of it remains today.
But Ronnie Tallon's cross had a different fate. While the large flags, wooden stage and associated structures were dismantled in the weeks after the event, the cross was left where it was and its future was uncertain.
There were few official calls to take it down, although state papers released 30 years later showed that the Office of Public Works favoured its removal. Then Taoiseach Charles Haughey apparently changed his mind after an intervention from Archbishop Dermot Ryan.
A year after the visit, newspapers were reporting that the cross was set to stay. One report pointed out that it had been vandalised on several occasions in its first year and its base had been festooned with graffiti. For many years now, the base has been protected by a circular cage.
The structure is now considered part of the 'furniture' of a park that was named one of the world's best city green spaces at the inaugural International Large Urban Parks Awards.
It remains a popular attraction for visitors and for the coachloads of tourists who visit every day - a legacy of the John Paul II visit is the large car park that's accessible to buses.
And it's a site that's popular with young families as there's always an ice-cream van in situ and generations of children like to roll down the grassy mounds that were constructed after it was decided that the cross would be staying for good on the site.
And Ronnie Tallon's cross continues to attract controversy, not least during this summer's heatwave when council staff were photographed washing it during a period of strict hosepipe bans.
"This work is deemed essential in advance of the substantial preparatory works required and the event itself as the cross has not being painted in over four years," an OPW spokesperson said at the time.
"The build works on site will commence at the end of July and the painting of the structure must be complete in advance of the start of the works. This event will be attended by approximately 500,000 people and viewed by millions from all around the world on television."
Meanwhile, Arcana's domed stage - reportedly inspired by the Claddagh Ring - shows off the cross prominently. Almost four decades on from its imagining in a Merrion Square architect's practice and its creation in an Inchicore steelworks, the cross continues to stand tall.
An exhibition on the visit of Pope John Paul II and Ronnie Tallon's central role in the event is on at the Phoenix Park Visitor Centre until September 30