County Mayo has a rich archaeological heritage dating from prehistoric times to the present. (Achaeology is the interpretation of our past from the study of buildings and objects made by human beings. We are dependent on archaeology alone in any attempt to study the prehistoric period and thereafter to complement what is recorded in written sources). According to the present state of archaeological knowledge, the first people arrived in Ireland sometime before 7000 BC during what is called the Mesolithic period. They were nomadic tribes of hunters and fishing people who built no permanent structures such as houses or tombs. The first colonisation of Mayo probably took place during that period.
In the fourth millennium BC, during the Neolithic period, another group of settlers arrived in Ireland, our first farmers, who introduced agriculture and animal husbandry to the country as well as the skills of pottery-making and weaving. They started a custom of burying their dead collectively (usually cremated) in large stone-built chambered tombs known as megalithic tombs, the earliest surviving architectural structures in the country. There are over 1,500 such tombs identified in Ireland with approximately 160 in County Mayo. This fact indicates the importance of the Mayo region during the Neolithic period and into the Bronze Age (c. 2000- 400 BC) when this phase of tomb-building came to an end.
In the literature on archaeology, Irish megalithic tombs are divided in four classes: court-tombs, portal tombs, passage-tomb and wedge-tombs, each style named after its chief diagnostic feature. Each class of tomb probably represents a new major colonisation of the country by different groups of tomb-builders. The remains of some megalithic tombs are so badly damaged that they can not be accurately identified by type and are consequently recorded as unclassified megalithic tombs. Examples of all types decorate the Mayo landscape. Eighty-five of the 400 plus court-tombs known in Ireland are located in Mayo. About 30 of these tombs are situated in the hinterland of Bunatrahir and Killala Bays in north Mayo. Others are scattered throught the county in the hinterland of Ballina and in places like Killasser in east Mayo, Ballycroy and Belmullet in the north-west, Claremorris, Cong, Achill, Newport and Louisburgh. There are seven known portal-tombs in the county (two in Ballyknock near Ballycastle; one in Claggan, near Ballycroy; one at Gortbrack North and another at Knocknalower, near Belmullet, one in Achill and another in Killasser); one identified passage-tomb at Carrowreagh near Bonniconlon (alias Bunnyconnellan) , with other possible ones in the Cong/Ballinrobe region. There are over 30 wedge-tombs and a similar number of unclassified megalithic tombs in the county.
The blanket bog which covers parts of Ireland developed from the late 3rd millennium BC onwards and in places covered the field systems, habitation-sites and tombs of the early farmers. Extensive pre-bog field-systems with stone walls have been discovered embedded in the bog in many parts of Ireland, notably at Behy, Glenulra and Belderrig , west of Ballycastle in County Mayo. The Behy/Glenulra region, known as the 'Céide Fields', contains a 1,500 hectare archaeological site, the most extensive Stone Age monument in the world.
Mayo has many known monuments from the Bronze Age: 34 wedge-tombs; 12 stone alignments/rows; 24 stone circles; close to 300 ancient cooking-sites known as fulachta fiadh. The county has also several monuments from the Early Iron Age (c. 400BC-AD 400): over 250 crannógs (lake-dwellings); over 100 promontory forts, and numerous ringforts and souterrains.