The initial incursion of pandemic (H1N1) 2009 influenza A virus (pH1N1) into a European pig population is reported. Diagnosis of swine influenza caused by pandemic virus was made during September 2009 following routine submission of samples for differential diagnosis of causative agents of respiratory disease, including influenza A virus. All four pigs (aged six weeks) submitted for investigation from a pig herd of approximately 5000 animals in Northern Ireland, experiencing acute-onset respiratory signs in finishing and growing pigs, were positive by immunofluorescence for influenza A. Follow-up analysis of lung tissue homogenates by real-time RT-PCR confirmed the presence of pH1N1. The virus was subsequently detected on two other premises in Northern Ireland; on one premises, detection followed the pre-export health certification testing of samples from pigs presumed to be subclinically infected as no clinical signs were apparent. None of the premises was linked to another epidemiologically. Sequencing of the haemagglutinin and neuraminidase genes revealed high nucleotide identity (>99.4 per cent) with other pH1N1s isolated from human beings. Genotypic analyses revealed all gene segments to be most closely related to those of contemporary pH1N1 viruses in human beings. It is concluded that all three outbreaks occurred independently, potentially as a result of transmission of the virus from human beings to pigs.