The way most Christians sing this ancient Irish hymn (translated by Mary Elizabeth Byrne [1881-1931] and versified by Eleanor Hull [1860-1935]) does not make much sense. I found that this version of the words from a British hymnal makes what the hymn is trying to say much more coherent.
Be thou my vision, O Lord of my heart,
be all else but naught to me, save that thou art;
be thou my best thought in the day and the night,
both waking and sleeping, thy presence my light.
Be thou my wisdom, be thou my true word,
be thou ever with me, and I with thee Lord;
be thou my great Father, and I thy true son;
be thou in me dwelling, and I with thee one.
Be thou my breastplate, my sword for the fight;
be thou my whole armor, be thou my true might;
be thou my soul’s shelter, be thou my strong tower:
O raise thou me heavenward, great Power of my power.
Riches I heed not, nor man’s empty praise:
be thou mine inheritance now and always;
be thou and thou only the first in my heart;
O Sovereign of heaven, my treasure thou art.
High King of heaven, thou heaven’s bright sun,
O grant me its joys after victory is won;
great Heart of my own heart, whatever befall,
still be thou my vision, O Ruler of all.
*Since you need a slightly altered version of SLANE to sing these words, I have linked what I believe to be a public domain scan to the correct setting.