I agree with Mooney's comment. The whole sonng is ironic...... with "Mother England" trying to keep control of her "naughty boys" whether they want it or no....

The words are not "From Bearna Baol to Bunker Hill", but "From Bhéal an Átha Bui to Pieter’s Hill". Bearna Bhaoil is in the National Anthem, but just means "Gap of Danger"... and why would BUnker Hill be referenced?

The first (Bhéal an Átha Bui) refers to the Battle of the Yellow Ford (1598), an Irish victory over English forces in the Nine Year's War.

The second (Pieter's Hill) is a reference to the battle of Collenso (1900) during the Boer War, where the Irish Transvaal Brigade, fighting for the Boers, caused serious casualties on the British side.