The fossil record of the Devonian tree Archaeopteris/Callixylon is extensive worldwide, however, the underground parts of these plants remain scarcely known. To date, there are only three studies that provide detailed anatomical descriptions of their roots. This study describes new anatomically preserved roots and stems of Callixylon from uppermost Famennian (Upper Devonian) deposits of Sandeel Bay, County Wexford, Ireland. The wood is characterized by tracheids with the pitting pattern typical of Callixylon (i.e., radial pits forming groups separated by non-pitted areas), and the presence of numerous ray tracheids that are smaller than the parenchyma ray cells in tangential section and unevenly arranged inside the rays. The roots are characterized by a three-lobed actinostele with multiple exarch protoxylem strands, an unusual organization reported for the first time in Callixylon roots. The stems are eustelic, with an heterocellular pith composed of thin and some thick-walled cells. Based on their unique combination of characters, the specimens are assigned to a new species, Callixylon seamrogia, the first species of Callixylon reported from Ireland. Based on comparisons with previous architectural studies of Callixylon, the stems are hypothesized to correspond to main axes of the new species, one of them bearing an apically emitted non-persistent branch. These new specimens from Ireland provide new information on the rooting system of Archaeopteris/Callixylon and improve our understanding of the anatomical and systematic diversity within the genus.