At about the same time Christianity arrived on the scene and closer contact with the world of the Romans was established. From then on technological advances spread and agriculture in Ireland, and thus food production, improved dramatically, leading to a substantial increase in the population.
This led to the construction of tens of thousands of ringforts or 'raths' - the remains of which were the subject of an archaeological dig in Newbuildings on Monday to Wednesday of last week.
"These raths provided the security a settlement needed at the time but towards the end of the first millennium the ringfort fell out of use. The reasons are lost in time and there are many suggestions as to why their popularity faded. One of these suggestions is that the arrival of the marauding Vikings meant the ringfort defences became obsolete since they'd moved warfare onto a different plane," said Roger McCorkell, a keen history and archaeology buff.
"It was at this time that, as the ringforts vanished, a new innovation in defensive architecture appeared - the souterrain, which we have been excavating here."
Souterraine
A souterrain is an underground structure, often built within pre-existing ringforts but many were constructed independently of enclosures and the most common are built in a dry-stone style.
They were made by digging an open trench which was then lined with dry-stone walls, roofed with large lintels and then covered with soil.
"The passages between the chambers are usually narrower and lower and the construction process frequently included an obstruction such as a hanging lintel which 'squeezed' the access space, thus reducing the opportunity for rapid or easy ingress - these are often referred to as 'creeps'. Other Irish souterrains show the use of blind alleys and split levels which were designed to confuse the intruder," he said.
According to Roger, the function of the souterrain has long been the subject of debate and the most accepted opinion is that they were used primarily as a place of refuge and occasionally as a storage site.
"It's perhaps harder to accept that these - hard to access - tunnels were where people stored their everyday goods but, seen as refuges from quick enemy raids, where the object would have been to capture people, they must have been effective. It would have been a very confident raider who would have been the first to crawl down a dark tunnel into the unknown with an unknown number of people, who knew the layout intimately, ahead of him in the dark," he said.
"Most of the history on this site dates to about 800AD to 1200AD, with the rath built first as a defensive enclosure, and they would have kept the animals in there at night. We have found evidence of the defence ditches right round the hill top, and after that came the Viking threat.
Vikings
"The Vikings would have come up the Foyle Estuary. They had an encampment on Lough Foyle somewhere around Magilligan, and they would have come up the Foyle in their longboats. The Vikings were looking for slaves and would have taken prisoners - women and children - and maybe if they had been coming up the river they would have seen smoke coming from the houses, so they would have tried to do a raid and get animals and residents for slaves.
"The rath dwellers would have seen them coming because this structure is built at a height, and if they saw the longboats they would leave the enclosure and run to these underground tunnels. These were just for people and they would have had a secret entrance."
Roger reveals that a rath of the size in Newbuildings would have had five or more souterrains, and said the dig was the culmination of a dream that he had had since he was a 15-year-old boy fascinated by history, and would not have been possible without National Lottery funding of £9,575, for which the group was very grateful.
Asked if he foresaw a day when he might excavate the whole site, he wistfully noted that it could take from £100,000 to £250,000 depending on how the job was done, and he would need ownership of the site.
"If you did excavate it, you would then have to cover it all up again and leave the entrance open, but after that, that would be all you would see," he adds.
Aleksandra
Among those taking part in the dig was Aleksandra Osinska, who is 26 years old, an MA graduate in archaeology, from the University of Stefan Wyszynski in Warsaw.
For the past year she has been living in Belfast and is now working for 'Gahan and Long' company, which is how she came to be involved in the dig last week.
"This is the first Rath I have seen, so I am very excited to be working here. It is in good condition for its age, it is not very damaged. I used to working on worse sites where there is more damage, this is good."
Asked what brought her to Northern Ireland, Aleksandra said she had been "looking for adventure".
"I have been here since Monday and I have been working in the ditch in the Rath as well as on the souterrain. Unfortunately, we have not found any small artefacts, we have only found the structure of the souterrain, but it is very exciting."