The 41-year-old detective was gunned down during a robbery at a credit union in Dundalk, Co Louth on Friday 25 January.
Police from Northern Ireland have teamed up with An Garda Siochana to track down the killers.
The Irish League of Credit Unions has offered a reward of €50,000 for information that will help the investigation.
It is believed the gang who carried out the murder travelled from Dundalk to Newtownhamilton, in Co Armagh, in a dark blue Volkswagen Passat.
The car was found abandoned and burnt out in Fews Forest, just 25 miles from the fatal attack.
A green and black Cosatto child's car seat which was in the Passat when it was stolen from Co Louth in the days before the attack has disappeared.
The booster seat was almost new, and Garda believe it may have been dumped, sold, or given to someone as a gift.
A red and black hammer which was found at the murder scene is being tested by forensics officers. It is thought the hammer would originally have been used by motorbike mechanics or panel beaters.
Anyone who is missing a hammer or has seen the child's car seat is asked to contact Garda.
Meanwhile, the PSNI are tracing the movements of the Passat around Newtownhamilton on the night of the murder, especially at the Chalybeate Road junction with Cortamlat Road, Cortmalat Road junction with Blaney Road, Keady Road junction with Chalybeate Road and Tullygeasy Road junction with Keady Road.
Police have appealed to anyone who saw the car or other cars in the area after 9.30pm or saw any vehicles on fire to speak to them.
Garda have investigated 2,300 lines of inquiry and interviewed more than 800 people in their hunt for Detective Donohoe's killers. They have also collected CCTV footage that teams will examine in detail.
Assistant Commissioner Kieran Kenny said: "On the night of the murder in the car park in Lordship Credit Union there were at least five people involved in the incident. One person fired the fatal shot that killed Adrian Donohoe
"There are people out there who have knowledge of Adrian's murder and I would ask them to reflect on the impact that Adrian's death has had on the family, the Garda Síochána and the community where he lived and where he served.
"I would appeal to anybody who has information about the murder to contact us please."