Operation Fanacht Garda checkpoints are in place across Donegal and Gardaí will cooperate with the PSNI at certain cross-border locations.
Since midnight Donegal has moved into Level 3 Covid-19 restrictions in response to rising case numbers.
It came after the county overtook Dublin as the area with the highest infection rate per 100,000 people at 148.2. Dublin currently stands at 145 cases per 100,000 of population.
Donegal is the fifth county to face temporary localised restrictions and the second, after Dublin, to enter Level three.
Under the new rules, residents will not be allowed leave the county unless absolutely necessary.
Gardai will also be on patrol in key areas of neighbouring Sligo and Leitrim.
The new restrictions will apply until October 16.