The recent political, economic and social instability in Zimbabwe has led to a dramatic increase in both legal and illegal migration to South Africa.
It is now estimated that between one to three million Zimbabweans are currently living in South Africa.
The most vulnerable of those on the move are the unaccompanied or separated migrant children from Zimbabwe who risk sexual exploitation, abuse and trafficking.
The journey to South Africa from Zimbabwe is fraught with all the dangers of illegal border crossing and Donncha found out how many children, especially girls, are at the mercy of bus operators, truck drivers and traffickers who smuggle them into the country.
Children as young as five years of age make the journey. They are usually in the company of teenaged friends or extended family members, but sometimes they get separated or are left to cope on their own.
Those who arrive unaccompanied typically have no form of documentation, making it difficult for them to obtain asylum.
To help protect these children, UNICEF is strengthening documentation and registration procedures for them and working to establish child-friendly safe spaces and special caregivers for the children.
Donncha's first stop on his field visit was to visit the Central Methodist School in Johannesburg.
The school was set up just over a year ago to cater to the needs of migrant children in central Johannesburg, who did not have access to schools in South Africa. Today, the school has over 500 students.
More than 100 students in the school are unaccompanied minors from Zimbabwe and live at the nearby Central Methodist Church.
The school provides these children with daily shelter, washing and cleaning facilities and three meals every day.
Although many of the students are from Zimbabwe, there are also students at the school from Mozambique, Malawi, Zambia and South Africa.
Donncha also visited the Central Methodist Church in Johannesburg. Bishop Paul Verryn opened the doors of this church to the migrant community in Johannesburg seeking shelter.
The church is now home to over 500 migrants seeking refuge.
The numbers of unaccompanied children at the church fluctuate continuously, with approximately 160 children currently living there presently.
UNICEF is working to provide services to the children living in the church, and at the same time, work with the South African Government to identify a new premises that can be developed specifically as a new home for the children.
Every day, child and youth care workers, trained from among the community at the church, ensure that this challenging environment is as child-friendly as possible and provides for the development of each individual child according to their specific needs.
Paying tribute to the children he met, the big Corkman said: "Many of the children I've met here have told me that the main reason they have come to South Africa is to continue with their education, following the collapse of the education system in Zimbabwe.
"It's very hard to come to terms with how these children live, away from their home, and who knows if they will ever be able to return.
"At the same time, it's amazing how resilient they are, how happy and ready to play and talk with us. They're amazing children."
Donncha also visited the township of Katlehong, where he met with UNICEF-supported child and youth care workers.
There are more children orphaned or left vulnerable by AIDS in South Africa than anywhere else in the world.
It is estimated that over half of the country's 2.5 million orphans have lost a parent to AIDS. As parents and extended family die or fall ill, it falls on the wider community to take care of the orphans left behind.
Donncha visited a very successful home-based support network - which is called the 'Isibindi Created Circles of Support Project', which is funded by UNICEF and reaches 33,000 children throughout South Africa.
Meeting and speaking directly with community child care workers, the Munster, Ireland and Lions lock gained first hand insight into how they work throughout their communities caring for and giving support to child-headed households and those that are headed by elderly grandparents.
Speaking about meeting with the families, he said: "It has been a life-changing experience for me to meet with children, who have to take on adult tasks such as caring for their sick parents and looking after younger brothers and sisters.
"It was also inspiring to meet with community workers who work with UNICEF's help, to guide and protect these children from harm and abuse.
"I now understand how HIV/AIDS has denied children their right to a secure home, their right to go to school - even their right to play.
"Seeing this first-hand has made me more determined than ever to do as much as I can to help UNICEF and I'm delighted to become an Ambassador for UNICEF here in Ireland."
Speaking about Donncha's appointment as a UNICEF Ireland Ambassador, Melanie Verwoerd, Executive Director of UNICEF Ireland, said that she was delighted that he had accepted the role.
"Donncha has already shown great commitment to UNICEF's work and made a fantastic impression on all the young people and staff he met during his visit in South Africa," she said.
"We look forward to working with Donncha to continue raising awareness and funding for our work worldwide for children."
To make a donation to support UNICEF's continuing work, please visit www.unicef.ie or telephone 1850 767 999. Postal donations can be sent to UNICEF Ireland, Freepost, 33 Lower Ormond Quay, Dublin 1.