'When Cannons are Roaring' is an English Patriotic song, it refers to battle and guns, so it's most likely referring to the 9 Years War, a war fought between the English and Irishmen between 1594-1603. It's lyrics refer to the blasting bombs 'pagans' and the bullets flying around the battle field.
The date of origin is debated, either 1624, or 1662, depending on the source. The elder one says it was written during the reign of James VI & I, after the same tune was first written that year and popular during the English Civil War, and the other says its first mention is from 'Cantus, Songs and Fancies' a 1662 sheet.
My take is that the song has existed since the 1620s, though poorly documented or having different names or verses, as figures from before the 1660s, such as Robert Munro, a Scottish general, mention it in a document in 1637. It's most likely an old war song that either wasn't documented in full for years, or an earlier full publication has gone missing, somewhere in the 1620s or 1630s.
Recording done by the band ‘The Druids’ I don’t own the music in any way.