Image copyright Getty Images Image caption An aerial photograph showing a trench system between Loos and Hulluch

A thick, cloudy, lethal gas crept across the murky, moon-like landscape.

This was no man's land. It was 27 April 1916, close to the village of Hulluch in northern France.

A slow moving gas attack would soon leave hundreds of entrenched Irishmen dead.

The Battle of Hulluch was an early example of the devastating effect of chemical warfare, then still in its infancy.

Yet the events at Hulluch are comparatively unknown and its significance is often overlooked.

Rebellion at Easter

Another reason the details of the gas attacks at Hulluch have been partially lost over time is that it took place at the same time as the Easter Rising in Dublin.

The rising would radically alter the history of Ireland, from where many of Hulluch's victims hailed.

While Dublin was in turmoil, Irish regiments such as the Royal Dublin Fusiliers, Royal Inniskilling Fusiliers and the Royal Irish Fusiliers were holed up across a vast network of trenches over 400 miles away on the plains of northern France.

Richard Bennett, Trustee at the Inniskillings Museum in Enniskillen, remarked that, when researching the history of the Inniskillings, "we came across information on the Battle of Hulluch which was fought at the same time as the events in Dublin. The battle was comparatively unknown and yet was of huge significance".

The Irishmen of the Inniskillings and other regiments, like many others across the United Kingdom, had signed up to fight against the Germans.

Unlike most others, they were overwhelmingly Catholic and nationalist, encouraged to sign up by Irish Parliamentary Party leader John Redmond.

Redmond urged his supporters to go, "wherever the firing line extends in defence of right, of freedom and religion in this war".

The gas attack

World War One saw the emergence of chemical warfare and it terrified soldiers more than any other weapon.

Sir John French, commander of the British Expeditionary Force, called the use of poison gas "a cynical and barbarous disregard of the well-known usages of civilised war".

It was used to kill, injure and demoralise soldiers and the gas attacks at Hulluch offered a terrible example of its ruthless efficiency.

Image copyright Getty Images Image caption Poison gas was used by both sides during World War One

Allied lines knew a gas attack was imminent. A deserter revealed plans of a German attack in advance and aerial reconnaissance had shown gas cylinders being readied.

After the Battle of Loos in 1915, where British forces had carried out gas attacks for the first time, they were well aware of the indiscriminate slaughter caused by chemical weapons.

As dawn broke on the morning of the 27 April 1916, the Royal Bavarian Corps of the Imperial German Army attacked the British trenches with a barrage of machine gun fire followed by heavy artillery bombardment.

Simultaneously, the German forces released 3,800 cylinders of chlorine and phosgene gas which drifted across no man's land.

Image copyright Corbis Image caption A British machine gun unit wearing their anti-gas PH helmets

The diary of Lt Colonel Edward Bellingham from County Louth describes how, by 5:30 BST on April 27, "a dense cloud of black gas and smoke" settled over the Irish lines followed by a heavy bombardment of the frontline trenches.

The effects were so intense, gas masks had to be worn as far as three miles behind the frontline.

Taking advantage of their opponents' reduced visibility, German troops stormed several Irish trenches where hand-to-hand combat ensued.

Although most of the casualties were caused by the subsequent German bombardment that destroyed the Irish trenches, it was the gas attack that created the conditions to make the artillery attack more deadly.

Valley of Death

The attack was devastating, particularly for the Irish.

Father William Doyle, a regiment chaplain, described what he saw in a letter home: "There they lay in the bottom of the trench, in every conceivable posture of human agony; the clothes torn off their bodies in a vain effort to breathe, while from end to end of that valley of death came one long unceasing moan from the lips of brave men fighting and struggling for life."

The conduct of the 7th Royal Inniskilling Fusiliers was commended by a war correspondent from The Times, who paid the tribute that "never was a job more quickly or more cleanly done."

Image copyright Impartial Reporter Image caption The Royal Inniskilling Fusiliers suffered heavy losses at Hulluch

Despite their bravery, the 7th Inniskillings suffered heavy losses.

Their commanding officer, Lt Colonel Young, said: "I desire to express to all ranks my high appreciation of their conduct and bearing, when they displayed a high standard of courage and endurance."

Faulty helmets

By January 1916, all British soldiers had been given anti-gas PH helmets to help protect them against the threat of poison gas.

The helmets consisted of layers of flannel cloth which had been dipped in a phenate-hexamine solution which, in theory, would protect the wearer from lethal chemicals.

The large number of deaths caused by gas attacks were commonly blamed on the soldiers for supposedly displaying poor discipline with their anti-gas uniform.

In fact, it was the poor manufacture of their helmets, along with the gas being of sufficient concentration to penetrate the masks, that led to such high levels of casualties.

When this was realised, production of the Small Box Respirator was accelerated as this had proven much more effective in combating gas poisoning.

'Heavy Uproar'

Two days after the initial attack, the day the Easter Rising came to an end in Dublin, the Germans attempted another gas attack in Hulluch.

Bellingham remarked in his diary that "the casualties from gas poisoning were more severe than on the 27th, owing presumably to the gas clouds meeting and remaining stationary and concentrated over the trenches".

This also resulted in many Germans troops being poisoned by their own gas as it lingered in the air.

Events in Ireland did not go unnoticed by the German troops.

They held up makeshift placards for their Irish adversaries: "Irishmen! Heavy uproar in Ireland. English guns are firing at your wifes and children." It was a vain attempt to get the Irishmen to turn on their fellow soldiers.

Image copyright Getty Images Image caption A handwritten sign found in the German trenches informing Irish troops of the Easter Rising in Dublin

As the centenary of the Easter Rising is marked by various commemorative events across Ireland, the attack at Hulluch remains relatively obscure, despite the higher number of fatalities.

In a matter of days, 570 were killed and many of the other casualties were to die later from respiratory diseases.

Like many battles during World War One, heavy losses were suffered for little military gain.