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Extraction of polyphenols from <i>Centranthus longiflorus</i> stems was conducted using ultrasound and infrared <i>Ired-Irrad</i>^® techniques, and compared to the conventional water bath method. Response surface methodology was used to analyse the effect of time, temperature, and ethanol percentage, as well as to optimize the three extraction methods. The highest phenolic content (81 mg GAE/g DM) and antioxidant activity (76% DPPH inhibition) were recorded with the <i>Ired-Irrad</i>^® extract obtained under the optimal conditions: 55 °C, 127 min, 48% (<i>v</i>/<i>v</i>) ethanol. Biological activities (antioxidant, antibacterial and antibiofilm) of the three extracts were assessed. All <i>C</i>. <i>longiflorus</i> stems extracts showed limited antibacterial effects regardless of the extraction method (MIC = 50 mg/mL), whereas <i>Ired-Irrad</i>^® extract exhibited the highest biofilm eradication and prevention capacities (93% against <i>Escherichia coli</i> and 97% against <i>Staphylococcus epidermidis</i>, respectively). This bioactivity is likely related to abundant caffeoylquinic acid and quercetin rutinoside, as identified by RP-UHPLC-PDA-MS analysis. The results obtained further promote the effectiveness of <i>Ired-Irrad</i>^® as a highly flexible and cost-efficient extraction technique.