Macroalgal phlorotannins and peptides are increasingly being investigated for their health-promoting properties; however in-depth research into the occurrence, bioactivity, profiling and molecular properties of these components in Irish species is limited. The process by which bioactives are extracted and purified from their original biomass must be efficient and food-friendly to be applicable to the food industry. Two different extraction methods, solid-liquid extraction (SLE) and pressurised liquid extraction (PLE), were compared for the extraction of polyphenols from three brown macroalgae, Ascophyllum nodosum, Pelvetia canaliculata and Fucus spiralis, and a green macroalga, Ulva intestinalis. SLE was deemed to be a more industrially relevant technique for generating food-friendly antioxidant extracts than PLE as it requires less capital investment, is food-safe, and can be scaled up. Molecular weight cut-off (MWCO) fractionation and reverse-phase flash chromatography techniques were employed to enrich the polyphenol content of brown macro algal SLE extracts by removing predominantly sugars from the extracts. Furthermore, the use of bioassay-guided fractionation indicated that the observed antioxidant activities of the extracts could be attributed to the polar, medium-to-high molecular weight phlorotannins. The analytical quality control and standardisation of naturally-sourced functional extracts has become a requirement for their use in the food industry. In this work, a rapid ultra-performance liquid chromatography® (UP LC®) method was presented for the profiling of low molecular weight phlorotannin isomers for polymers up to 16 monomers in length from the three brown species. The predominant size ranges of the phlorotannins found in A. nodosum, P. canaliculata and F. spiralis were 6-11, 6-13 and 4-6 monomers, respectively. The identification of bioactive peptides has been largely unexplored. In this work, various peptides were isolated from a trypsin-digested Ascophyllum nodosum protein extract. A selection of peptides were synthesised and their in vitro antioxidant and antihypertensive activities assayed. The polypeptide EKTGLLNVVETAEKFL displayed the highest renin enzyme inhibition of 56.12 ± 2.66 % and this is the first report of renin inhibitory peptides identified from brown macro algae Computational chemistry may act as a support to natural products experiment, through the provision of data on the molecular properties of individual components and also on potential binding interactions between mixtures of bioactive components. Density functional theory was employed to theoretically determine the radical scavenging potential of various 7-phloroeckol conformers through the calculation of their O-H bond dissociation enthalpies. Furthermore, a molecular dynamics simulation approach was proposed for the investigation of binding interactions between a selected macro algal phlorotannin and a bioactive peptide from A. nodosum.