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Border Control Checks operate on entry to Republic of Ireland and Northern Ireland with airlines and ferries and more so upon re-entry to Great Britain. Travel between the Irish Republic and Northern Ireland is usually hassle free but sometimes checks are made at the main line bus and train stations at Belfast and Dublin and Dundalk (ROI) and Newry (NI). Car travel between Belfast and Dublin and other routes normally tends to be hassle free unless there is security/smuggling/drugs checks. PSNI and Garda Siochona police forces cooperate on suspected national and international criminal activity.