Living (LM) and non-living maerl (NLM) rhodoliths of the species Lithothamnion corallioides (Crouan & Crouan, 1867) from Kingstown Bay, Galway, were sampled and compared in relation to their physical structure and lipid and low molecular weight carbohydrate (LMWC) composition. Saturated (SATFA) and polyunsaturated fatty acids (PUFA) were dominant, in particular 16:0, 20:4n-6 and 20:5n-3, but a diverse range of fatty acids were identified. The abundance of n-alkanals was high, and sterol composition was simple, with cholesterol accounting for over 90% of the total sterols. Mono-, di-, and trisaccharides, with galactose units being dominant, and floridoside were present in high abundance. Notably, the fatty acid and LMWC profiles varied little between NLM and LM. The relatively high abundance of PUFA and floridoside, in particular, suggests that NLM may have further potential for research and commercial purposes in a variety of food, biomedical and industrial applications. Previously reported unidentified ‘globular inclusions’ were more abundant in NLM and exhibited a crystalline morphology. Together with the bacterial fatty acid composition of LM and NLM, the results indicate that these structures are not bacterial in nature.