"The first, the children or subjects of Danu, were the Irish pantheon, a divine society of beings associated with each other and dwelling in a parallel world with its own politics and customs. They had individual functions, such as healer, smith, wheelwright, metalworker, harper and poet, suggesting that they may have functioned as patrons of people engaged in these activities. But there were also two who were multi-talented and occupy a higher status in the stories. One was Lugh, known as ‘the many-skilled’, ‘the long-armed’ or ‘of the long spear’. The first epithet gives away his nature. He was the sophisticated, inventive, brilliantly clever and handsome god, the favourite deity in the stories. He was the particular patron of heroes, and gave his name to the most joyous Irish festival, Lughnasadh. He seems to have been a very widespread deity in the Celtic world. His name appears in a Welsh tale, as Lleu Llaw Gyffes, ‘The Bright One of the Skilful Hand’, although the character concerned is not a god…It would be easy to imagine how such an attractive divine personality could have a very widespread appeal, but we do not know whether he had exactly the same identity upon the Continent as in Ireland…
…In the seventeenth century a myth was concoted that the so-called Teltown marriages’, trial weddings transacted for a set period at the fair of that name in County Meath, were originally ceremonies held at Lughnasadh and associated with the goddess Tailtu. In the 1950s this was disproved, it seems conclusively, and it looks as if ‘Queen Tailtu, foster-mother of Lugh’ was herself an early medieval poetic invention…
Marie MacNiell [has a] famous book upon Lughnasadh. She established that in the early medieval texts it was regarded as the celebration of the beginning of the harvest. She then located 195 sites, usually on heights or beside water, at which Irish villagers had celebration. She discovered that many of these places were associated with a local myth in which a heroic newcomer (normally St Patrick) had defeated an established and unpleasant lord (normally Chrom Dubh). She found memories of similar gatherings at this time in the Isle of Man, Wales and Cornwall. From all this she argued that Lughnasadh had been a festival held all over the British Isles, at which people assembled to mark the safe arrival of the time of harvest and the season of plenty. She further suggested that these gatherings had enjoyed a ritual performance, a story or a piece of drama in which the god Lugh defeated Chrom Dubh, symbolizing the conquest of the old god of the earth and his surrender of the harvest. Now, one firm conclusion which can be drawn from this marvellous work is that popular assemblies were indeed held in pagan Celtic times all over Island and Western Britain, and perhaps elsewhere in Britain, to celebrate Lughnasadh. The rest of the author’s reasoning is speculative, though legitimate and fascinating…
Lughnasadh, as already pointed out, has left copious traces of outdoor gatherings in Ireland, and some along western Britian, though the latter are few compared with those of Samhain. the problem here is that the Anglo-Saxons had their own festival to open the harvest, Loaf-mass or Lammas, which fell on the same day and was celebrated with fairs and gatherings which sometimes make it very hard to distinguish from the Celtic feast.”
From The Pagan Religions of the Ancient British Isles: Their Nature and Legacy by Ronald Hutton
Here is a fragmented poem for Lugus at Lughnasad, written and translated by Logodaedalus in Romano-Brythonic and English:
Nu garyū menwanē, I now call to mind,
saneston rosenon, an ancient tale,
Nowiyū ad·wēdū Told anew,
sugaryūs suwreχtūs. with well-wrought words.
Ro·cuclowa cantlā, I have heard songs,
ambi clowos Lugous. of Lugus’ fame.
Dercon daweti·yo, His eye that burns,
dīgalī nemesos. with heaven’s vengeance.
Wlidubi wlaticon At the feasts of kings,
Ro·wāta cowidwā, I have told tales,
Ganon are glanon, Of his birth by the shore,
gabaglan wogaisī, his weapon-winning.
Nu·c canū calmiyon, I sing of the skilful one,
cailācos altiyos. the foretold fosterling,
Com·yo berte messun, When the harvest was taken,
braton en magesi. treachery in the fields.
Here are some tips on celebrating this Iron-Age Celtic and Medieval Celtic festival from Land, Sea, and Sky by Francine Nicholson: