Irish edible brown (Himanthalia elongata-sea spaghetti, Alaria esculenta-Irish wakame) and red seaweeds (Palmaria palmata-dulse, Porphyra umbilicalis-nori) were assessed for nutritional (proximate composition; salt; pH; amino acid; mineral and dietary fibre contents); bioactive (total phenolic content (TPC) and in vitro antioxidant activity (DPPH and FRAP)); thermal (thermogravimetric analysis (TGA)); and technological (water holding capacity (WHC), oil holding capacity (OHC) and swelling capacity (SC)) properties. Red seaweeds had higher (p < 0.05) protein levels, whereas brown seaweeds possessed higher (p < 0.05) moisture, ash, insoluble and total dietary fibre contents. Nori had the lowest (p < 0.05) salt level. Seaweed fat levels ranged from 1 to 2% DW. Aspartic and glutamic acids were the most abundant amino acids. The total amino acid (TAA) content ranged from 4.44 to 31.80%. Seaweeds contained numerous macro (e.g., Na) and trace minerals. The TPC, DPPH and FRAP activities followed the order: sea spaghetti ≥ nori > Irish wakame > dulse (p < 0.05). TGA indicated maximum weight loss at 250 °C. Dulse had the lowest (p < 0.05) WHC and SC properties. Dulse and nori had higher (p < 0.05) OHC than the brown seaweeds. Results demonstrate the potential of seaweeds as functional food product ingredients.