From A History of Ireland by Thomas Mooney, published 1845. Source of the text is taken from the Annals of Inisfallen, the speech itself is likely to be apocryphal.

Be not dismayed, my soldiers, because my son Donough is avenging our wrongs in Leinster; he will return victorious, and in the glory of his conquests you shall share. On your valour rest the hopes of your country to-day; and what surer grounds can they rest upon? Oppression now attempts to bend you down to servility; will you not burst its chains, and rise to the independence of Irish freemen? Your cause is one approved by Heaven; you seek not the oppression of others; you fight for your country and your sacred altars. It is a cause that claims a heavenly protection. In this day’s battle, the interposition of that God who can give victory will be signally manifested in your favour. Let every heart, then, be the throne of confidence and courage. You know that the Danes are strangers to religion and humanity; they are inflamed with the desire of violating the fairest daughters of this land of beauty, and enriching themselves with the spoils of sacrilege and plunder. The barbarians have impiously fixed, for their struggle to enslave us, upon the very day on which the Redeemer of the world was crucified; victory they shall not have. From such brave soldiers as you they can never wrest it; for you fight in defence of honour, liberty, and religion – in defence of the sacred temples of the Deity, and of your sisters, wives, and daughters. Such a holy cause must be the cause of God, who will deliver your enemies, this day, into your hands. Onward, then, for your country and your sacred altars!