Trinity Sunday at FHD

Full Homely Divinity has a new page about Trinity Sunday; some really nice Celtic poetic prayers can be found there.

For instance, "Michael the Victorious":

THOU Michael the victorious,
I make my circuit under thy shield,
Thou Michael of the white steed,
And of the bright brilliant blades,
Conqueror of the dragon,
Be thou at my back,
Thou ranger of the heavens,
Thou warrior of the King of all,
O Michael the victorious,
My pride and my guide,
O Michael the victorious,
The glory of mine eye.

I make my circuit
In the fellowship of my saint,
On the machair, on the meadow,
On the cold heathery hill;
Though I should travel ocean
And the hard globe of the world
No harm can e’er befall me
’Neath the shelter of thy shield;
O Michael the victorious,
Jewel of my heart,
O Michael the victorious,
God's shepherd thou art.

Be the sacred Three of Glory
Aye at peace with me,
With my horses, with my cattle,
With my woolly sheep in flocks.
With the crops growing in the field
Or ripening in the sheaf,
On the machair, on the moor,
In cole, in heap, or stack.
Every thing on high or low,
Every furnishing and flock,
Belong to the holy Triune of glory,
And to Michael the victorious.

Not necessarily a "chant" post - unless you put St. Patrick's Breastplate in that category. (Which I think you could....)