Ducking down, they witnessed a soldier from the Dublin Fusiliers shot dead.
It was the start of the Easter Rising - a bloody armed insurrection against the British government in Ireland during Easter Week, 1916.
Now, a century later a historian has uncovered the forgotten New Zealand connection to one of Ireland's most infamous episodes.
Ancestry's content director Ben Mercer stumbled across the link while researching its vast archival military records.
"In many accounts of the Easter Rising, the soldiers are simply referred to as ANZACs, and were assumed to have been Australian," Mr Mercer said.
"It's only through careful research into what the records tell us that we were able to gain a greater understanding of the soldiers' role in this important historical event."
Dunedin corporal Alexander Don, 21, had been recovering from dysentery in a Belfast hospital when he visited Dublin while on leave.
The veteran of the disastrous Gallipoli campaign was walking past Dublin Castle when he heard gunshots.
Two British troops fell in front of him.
"I thought I must be dreaming, and went over to where they were lying, and saw that one had got it through the head and the other through the neck," Don would later write in a letter to his father, Rev Alexander Don, a Presbyterian minister living in Roslyn, Dunedin.
Two rooftop Irish Republican snipers sized up his British Imperial Army uniform.
But they hesitated, confused by his New Zealand 'lemon-squeezer' slouch hat which was so similar to their own.
Local women gave Don civilian clothes, allowing him to explore the mayhem.
"At the Post Office, I saw the Commander-in-Chief, [James] Connelly, standing with folded arms in a doorway. I could have shot him from the crowd, but should have got two or three into me, as rifles and green could be seen from every window," he said.
He then encountered two New Zealanders - fellow Gallipoli survivors, Whangarei blacksmith Edward Waring, 23, and 21-year-old Milton mechanic Finlay McLeod.
As the trio walked past Trinity College, the country's oldest university dating back to 1592, a porter unlocked the gates and hurriedly let them in.
They were joined by Garland and Nevin who had reported to the college earlier.
The five Kiwis were marshalled by British officers to help defend Trinity College from the Irish republican soldiers.
They were armed and stationed to the rooftop to shoot to pick off rebel snipers.
Thoughts of a peaceful few days away from the frontline were now a distant memory as they were surrounded by marauding rebels.
"The Kiwi soldiers were completely caught by surprise," Mr Mercer said.
"Leave passes gave thousands of soldiers the opportunity to travel throughout the United Kingdom, which at the time included all of Ireland. It was a bit of a boys-own adventure, something comparable with today's 'Big OE'.
"The Kiwis felt far removed from the horrors of the battlefield and were expecting a weekend of sightseeing, not the bloody chaos that erupted around them."
The five New Zealanders - along with an Australian, two Canadians and five South Africans - remained on the rooftop from midnight Easter Monday until Thursday, living on biscuits and water, with just snippets of sleep caught between shootouts.
Their first action came at around 4am on Tuesday morning when three Sinn Fein soldiers biked past.
"We all fired at once, killing two and wounding the other," Garland wrote home to his father, Thomas Garland in Mt Eden, Auckland.
One rebel was shot in the head four times but despite being killed instantly "continued on for about 30 yards on the free-wheel", Garland recalled.
Over the next few days, the rooftop Anzacs claimed responsibility for 27 rebel deaths, including three women.
Garland caught a ricochet bullet in the left ankle and earlier while re-capturing a railway station a "mere scratch" from a bayonet.
"Several of the chaps from Gallipoli reckon that one had a far better chance of getting off with his life there than in the Dublin riot," he wrote.
After the Easter Rising, the New Zealand soldiers sailed back to England to rejoin their units.
They all survived action on the Western Front and returned home.
However, Waring succumbed to Spanish Flu in November 1918, aged 26.
Army service files make no mention of their unofficial 'active service' in Dublin.
But in August of 1916, each Kiwi soldier was sent a small silver cup by the board of Trinity College, recognising their part in the brave defence of the university.
"This recognition would have provided them with a great deal of gratification that they'd been recognised for their valour and for risking their lives," Mr Mercer said.
• To commemorate the centenary of the 1916 Easter Rising, Ancestry is releasing records directly linked to the rebellion: http://search.ancestry.com.au/search/group/easter_rising_records
