Sven's notes are found in his article The Druids and the Early British Church
The following notes represent the citations from various books quoted by William Finck during the audio presentation. The writings of Bede, Gildas and Nennius are found in the References section here at Christogenea.
From The Annals of Ireland translated from The Original Irish of The Four Masters by Owen Connellan in 1846, from a footnote on the Druids found on page 75:
About nine centuries before the Christian era, according to our ancient annalists, Tigearnmas, monarch of Ireland, of the race of Heremon, was the ﬁrst who introduced Druidism and the worship of idols into Ireland; and it is stated, that while worshipping the idol Crom Cruach, the chief deity of the Irish Druids, along with a vast assemblage of his subjects at Magh Sleacht in Breifne, on the feast of Samhuin, (one of their deities, the day dedicated to whose rites was the same as the last day of October), he himself, with three-fourths of his people, were struck dead by lightning, as a punishment from heaven for his introduction of idolatry into the kingdom. Magh Sleachta signiﬁes either the Plain of Adoration, or the Plain of Slaughter, and obtained its name from the Druidical rites performed there, or from the human sacriﬁces which the Pagan Irish offered up to the deities of Druidism, as the Canaanites offered up their’s to Moloch. In this place stood a famous temple of the Druids, with the great idol Crom Cruach surrounded by twelve minor idols, composed of pillar stones, and decorated with heads of gold....