Eamonn of the Marshes (I) is the husband of Eilis (I), son-in-law to Seamus Redbeard and father of Eamonn (II) and Aisling. He is mentioned briefly in both Daughter of the Forest and Son of the Shadows. He is the Irish Lord of Caiseal Dubh - an area east to the lands owned by Seamus Redbeard (Glencarnagh) and his home is identified by the expanse of the notorious marshlands.
Eamonn of the Marshes marries Eilis, who was once engaged to Liam. He is noted for his betrayal of his alliance of the other Irish lords - Lord Colum and Seamus Redbeard - when he gives special intelligence and men to Lord Richard, the British ruler of Northwood. He also provides Richard with the wood for Sorcha's burning which burns the flesh from the bone while sparking colours of yellow, green and red. It is implied that Eamonn has an affair with the Lady Oonagh while she stays at his Caiseal Dubh.
When Sorcha's brothers recover from their transformation and return home, Sorcha passes on the information about Eamonn's betrayal. Although it is not disclosed in the series, it is alluded that Eamonn of the Marshes is removed from power by Seamus Redbeard, Lord Colum and his sons for his treachery. He is succeeded by his heir, Eamonn (II) who assumedly takes after him in looks, having straight, chestnut brown hair. He inherits his father's reputation and devious, vicious streak as well as his inclination for betrayal.