Shortly after his arrival he addressed a memorial to the lord-lieutenant, the Earl of Camden, on the disturbed state of Ireland, advocating the formation of a strong military force, composed impartially of Catholics and Protestants.

On the rumour of Hoche's expedition in 1796 he accepted a commission as aide-de-camp to General Dundas; but, becoming disgusted at the violent measures of government, he became in November of that year a United Irishman.

Morres managed to escape from Dublin on 4 June, and lay concealed in co. Westmeath till the arrival of Jean Joseph Amable Humbert's expedition on 22 August.

He was cordially welcomed, as an old friend of her husband, by Lady Fitzgerald; but, having been included by name in the Rebel Fugitives Act, he did not feel secure in Hamburg, and applied to the French resident, Marragon, for permission to proceed to France.

After ten months' close confinement the senate of Hamburg consented to his extradition, and at midnight on 28 September 1799 he was, with his three companions, conveyed on board an English frigate at Cuxhaven.

The subserviency of the senate of Hamburg caused universal indignation, and drew down upon them Napoleon's wrath, which was only appeased by the payment of a fine of four millions and a half francs and a public apology.

Morres pleaded that he had been arrested eight days before the time assigned by the act for his voluntary surrender had expired, and, after a long argument, his objection was sustained by Lord Kilwarden.

His wife having died at the age of twenty-six, on the very day of his arrest at Hamburg, Morres, after a brief visit to Paris, married, at Dublin, Helen, widow of Dr. John Esmonde, hanged as a traitor in 1798, and daughter of Bartholomew O'Neill-Callan of Osbertstown House, co. Kildare.

Some futile efforts were made by his family to induce him to return to Ireland, and his offer, after the abdication of Napoleon, to serve under the English flag not meeting with a cordial response from Wellington and Castlereagh, he retained his commission in the French army, and on 3 November 1816 he obtained letters of naturalisation.