The hymn is regularly cited as evidence that Irish Christianity absorbed native pagan religion, and the invocation of sun, fire, wind and sea is what the claim rests on. The claim is worth handling carefully.
Against it: the lorica genre is not distinctively Irish, similar compositions exist in Latin, and invoking the created order as testimony to and instrument of God is thoroughly biblical, the Song of the Three Children in Daniel does exactly this, calling on sun and moon, winds, fire, frost and every element to bless the Lord, and it was a standard liturgical canticle. The hymn's elements are creatures, not powers, and the theology is orthodox.
Also against it: the hymn explicitly lists druids among the dangers it guards against, alongside demons. That is not a friendly absorption of native religion; it is a defensive posture toward it.
For it: the text does show an early Irish Christianity comfortable with a poetic register that later Latin Christianity found strange, and the genre's popularity in Ireland and Wales is real.
The wider claim, that a distinctively "Celtic Christianity", nature-loving and unlike Rome, existed as an alternative tradition, is a modern construction assembled largely in the nineteenth and twentieth centuries, and historians of early medieval Ireland have dismantled it in detail. The Irish church was Roman in doctrine and argued with Rome mainly about the date of Easter and the shape of tonsure.
The best-known English version, sung to two Irish tunes, is Cecil Frances Alexander's of 1889.