Fourwinds,
From my friend Prof Vance Tiede,

The Irish "duibrey" is the "Black or Dark Moon" that occurs every 18 or 19 years when the midsummer full moon (declination -28d) is so low in the sky that elevated terrain to the south can mask it, and it does not appear above the skyline. As I wrote in my thesis on the solar alignments of Irish Early Christian oratories:

The modern Irish phrase, oiche dhuibhre (night of the black moon ), resonates with the ancient astronomy of Ireland's megaliths. (Dinneen 1927, 377) Archaeoastronomer John Barber reports the following conversation with local farmers who spied his surveying equipment standing in the middle of a 3,000 year old stone circle:

So you're interested in the Duibrey [dhuibhre], eh?
''What's the Duibrey?' asked Barber. 'Why, that's every nineteen years when the moon stays below the mountains.' (Hawkins, 1983b, 89-90)

In 1973, Barber published a computerized analysis demonstrating that recumbent stone circles in Counties Kerry and Cork were oriented to the setting position of the full moon at midsummer major standstill (-29° declination). (Burl 1976) The lunisolar concept embodied in the Irish expression of oiche dhuibhre is known among astronomers as the 18.6 year cycle. It refers to the full moon's spiral path between solstices. Whereas lunisolar orientation has been discovered only recently at megalithic structures in the British Isles, at least one linguistic link, the oiche dhuibhre, may have survived since pagan times. (Barber 1973, 26-39) Then as now, every 18 or 19 years,

...winter nights are brighter and moonlight lasts longer. In the summer the full moon skims low on the southern horizon.... If an ancient culture looked on the moon as important either in agriculture or hunting, fertility or death rites, the 18.6 year cycle...would ultimately be discovered. (Hawkins 1983b, 82)

2006 is the Year of the Midwinter High Moon and midsummer oiche dhuibhre.