Without more formal controls the Irish border is a relatively open immigration route. It is policed under “Operation Gull”, a long-running, intelligence-led initiative operated by the UK Home Office, the Police Service of Northern Ireland (PSNI), Police Scotland, and Ireland’s police force, the Garda Síochána.
Source: https://www.bbc.com/news/articles/crr89k8kp27o?at_medium=RSS&at_campaign=rss