Death of a POW at Valenciennes

Hi all , this is my first post so apologies for any errors ,thanks to a grave in my local cemetery dedicated to the wife of a former soldier I am trying to research a man called Thomas Mcdermott ( sometimes spelt Macdermott) . He was a militia officer in Ireland who joined the Irish Brigade in France in 1786, Like many royalist officers joined the ill fated Irish brigade in the British army,

He appears on the half pay list and in 1801 is listed as a colonel, the next record I found of him is as a POW in Valenciennes in 1806 where he died in 1809,

I would like to try find out more about his service in the period of 1800-1806 and also if possible find out where he is buried ? Any suggestions most welcome.

Ive tried Newspapers.com , the National archive , British Newspaper archive and findmypast .

Thanks in advance.