If you have obtained the sheet music, here are three essential tips to make it sound authentic—not just like a tin whistle with a bag:
James Horner composed the theme using the (specifically B Dorian). This is a "minor" sounding scale that is prevalent in Celtic folk music. It avoids the cheesy, happy resolution of a standard Major scale, giving the tune a somber, ancient quality. outlawed tunes on outlawed pipes sheet music
Composed for the Braveheart soundtrack, this track is primarily performed on the (Irish pipes) by soloist Eric Rigler . If you have obtained the sheet music, here