Although tunes may be similar and shuffled around to different sets of lyrics, "St. Patrick's Breastplate" is actually a prayer (supposedly written by St. Patrick himself) that have been used with different tunes. The song words I posted above are from 1920, by Sister Agnes. The prayer is called a 'breastplate' because it is meant to be a 'sheild' from harm.
There are different versions of the prayer, especially when it is used with a tune, but here is one:

St. Patrick's Breastplate
Christ be with me, Christ within me,
Christ behind me, Christ before me,
Christ beside me, Christ to win me,
Christ to comfort and restore me,
Chrst beneath me, Christ above me,
Christ in hearts of all that love me,

My great grandmother used to say this prayer.