The riddle of the inscriptions at Newgrange, Knowth and other equally ancient Irish sites in the Boyne valley has been partly deciphered at last. The inscribed passage mound stones tell of prehistoric man's concept of the world; the flat earth with a hemispherical bowl overhead, the sun and the moon circling round. The legends and myths of Ireland can be directly related to the stone engravings; certain numbers such as nine, eleven, seventeen, twenty-seven and thirty-three are common to both. These numbers have important symbolic meanings as well as their numerical values. The oldest calendar in the history of mankind is portrayed - sixteen months of 22 or 23 days, four weeks of five days each month, eight annual solar and seasonal events. It has been known for some time that the passages into the Newgrange and Knowth mounds are aligned with sunrise and sunset on the solstitial and equinoctial days each year. They are the cornerstones of the sixteen month calendar and the eight annual festival days. The Irish evidence from 3500 BC to 3200 BC precedes British calendar building sites at Mount Pleasant 2600 BC and Stonehenge 2000 BC. Neil L. Thomas is a chartered engineer who has worked in the oil and gas industry for over twenty-five years. Field trips to Ireland, Wales, England and Scandinavia have stimulated his researches on archaeological matters. He has endeavoured to re-create in his mind the ways in which early man would have tackled a problem and the likely answers he would have obtained. His other work includes Stonehenge Sunsets and Thirty-Three, British Neolitic Calendar Buildings and The Creation of Ancient Avebury,.