He is wounded and dying, writing a moving final letter to his loved ones.
But he is not fighting and he is not in a war.
Piaras is, in fact, only 11 years old, and he is portraying the last hours of Michael Joseph O'Rahilly, killed while taking part in the 1916 Easter Rising.
The drama "The Death of The O'Rahilly" is just one of the ways Holy Cross Boys Primary School in Ardoyne in north Belfast is marking the centenary.
The school has devised a special 1916-2016 education programme, placing the rising at the centre of their curriculum.
They have been teaching special lessons in geography, music, art and English.
But it was O'Rahilly's story, and letter, which captivated Piaras.
"It is a myth that he wrote it in his own blood," he said.
"But by the time the letter was delivered, he was already dead."
The school's short drama now has a chance of being performed on the stage of the Abbey Theatre in Dublin.
They have been shortlisted as one of only six schools across Ireland in a competition run by the famous theatre.
Meanwhile, in another classroom, P4 to P7 boys who are members of the school council are updating the 1916 proclamation.
Their 'Proclamation for a New Generation' takes the form of Padraig Pearse's famous document, but updates it for the 21st century.
It contains hopes for "a good job and good health care" and a commitment that "Ireland will be litter free".
"It does not matter what gender you are, what colour your skin is, what religion you believe in," it reads.
"All children of the nation will be cherished equally.
"Our Irish traditions, language and sport will be passed on to our children and grandchildren."
The proclamation will be revealed to the whole school at a special assembly in mid-March.
Elsewhere, pupils have been learning the names and locations of Ireland's 32 counties in geography, while others have been composing a song on the theme of 1916.
But is there a danger the focus on the rising could lead to indoctrination rather than education?
It is concern answered by the school's vice-principal Chris Donnelly, who said they take a rounded approach to history.
"What we're about is teaching across the length and breadth of historical issues," he said.
"In this school we teach the world wars, and the boys in primary four go to the Northern Ireland war museum every year.
"The principal has taken boys to meet Queen Elizabeth when she came here.
"The Easter Rising was a significant event in Irish history and therefore it does need to be talked about and taught.
"The boys will be aware from the community and what they see around them that it's the 100th anniversary.
"We're using that theme in a forward looking sense with the proclamation for a new generation."

"The president cannot suffer bright lights," George Charamba was quoted as saying by the state-run Herald newspaper.
Mr Mugabe, 93, has been seen apparently sleeping at several events, leading to speculation about his fitness.
He intends to stand in presidential elections next year.
The president is currently receiving specialised medical treatment for his eyes in Singapore.
Africa Live: More on this and other stories
Mr Mugabe most recently appeared to fall asleep during a discussion panel about "fragile states" at a World Economic Forum meeting in South Africa earlier this month.
There he stated that Zimbabwe was 'one of the most developed countries in Africa'
"I feel like a failure when there is this reading that the president is sleeping in conferences - no," Mr Charamba said.
He then compared Mr Mugabe to the anti-apartheid icon Nelson Mandela, whose eyes were affected by years of working in a limestone quarry while imprisoned on Robben Island.
"You were not allowed to even use flashes whenever he was in the room," Mr Charamba said.
Piaras Millar sits before me, bloodied and bruised, in the green uniform of a soldier.
[NEXT_CONCEPT]
Zimbabwe's President Robert Mugabe is not asleep when he closes his eyes for long periods during meetings but is resting his eyes, his spokesman says.