The fighting Irish . . . this is a quote from the famous Irish mythical epic Táin Bó Cuailnge. The Ulster heroes declare this to king Conchubhar when he calls upon them to leave a battle in order to meet an attack in another part of the field.
Heaven is above us,
and earth beneath us,
and the sea is round about us.
Unless the sky shall fall with its showers of stars on the
ground where we are camped,
or unless the earth shall be
rent by an earthquake,
or unless the waves of the blue sea
come over the forests of the living world,
we shall not give ground.
Read the Táin Bó Cuailnge here.