Probably too late for your tatoo but there is a large collection of Gaelic hymns and incantations from the highlands and islands of Scotland, collected by Alexander Carmichael in the 19th Century. Known as the Carmina Gadelica, it is published in English by Floris Books, Edinburgh, 1992. Pages 209 to 222 have prayers for protection.

There is also the Lorica (brestplate or shield) of St Patrick. Here are some of the lines from one translation:

I bind unto myself today
The power of God to hold and lead,
His eye to watch, His might to stay,
His ear to hearken to my need.
The wisdom of my God to teach,
His hand to guide, His shield to ward;
The word of God to give me speech,
His heavenly host to be my guard.

Against the demon snares of sin,
The vice that gives temptation force,
The natural lusts that war within,
The hostile men that mar my course;
Or few or many, far or nigh,
In every place and in all hours,
Against their fierce hostility
I bind to me these holy powers.

Against all Satan’s spells and wiles,
Against false words of heresy,
Against the knowledge that defiles,
Against the heart’s idolatry,
Against the wizard’s evil craft,
Against the death wound and the burning,
The choking wave, the poisoned shaft,
Protect me, Christ, till Thy returning.

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.
Christ beneath me, Christ above me,
Christ in quiet, Christ in danger,
Christ in hearts of all that love me,
Christ in mouth of friend and stranger.

I bind unto myself the Name,
The strong Name of the Trinity,
By invocation of the same,
The Three in One and One in Three.
By Whom all nature hath creation,
Eternal Father, Spirit, Word:
Praise to the Lord of my salvation,
Salvation is of Christ the Lord.

Iain