The 41-year-old father of two was fatally shot while he was escorting a cash delivery van to a credit union in the rural village of Lordship on Friday 25 January around 9.30pm.
Detective Donohoe was the first serving police officer to be killed in the Republic in 17 years. He and his wife Caroline were both officers at Dundalk Garda Station.
Two days after the shooting, a Volkswagen Passat was found in Fews Forest near Newtownhamilton in Co Armagh, about 25 miles away from the scene.
Police believe the gang involved travelled in the dark coloured car from Dundalk across the border to the forest, where it was abandoned and set on fire.
In an appeal issued on Saturday, the PSNI confirmed that the car is "still the subject of rigorous and extensive forensic examination in Northern Ireland."
Checkpoints were mounted by the PSNI at various locations near Newtownhamilton on Friday night.
The checkpoints were staged at the Chalybeate Road junction with Cortamlat Road, the Cortmalat Road junction with Blaney Road, the Keady Road junction with Chalybeate Road, and the Tullygeasy Road junction with Keady Road.
The PSNI are specifically appealing for anyone:
- who saw a dark coloured Volkswagen Passat in the area on the Friday night of 25 January after 9.30pm;
- who saw any other vehicles in the area around this time;
- who observed fire, smoke or burning vehicles in the area;
- or anyone who can assist in any other way to come forward.
Anyone with any information is asked to contact detectives in PSNI on 0845 600 8000 or detectives at An Garda Síochána on 042 9388470 in the Republic of Ireland.