| December 1939 | CO: Lt Col Sir Ronald Ross |
| Dec 39 | North Irish Horse |
| 2/12/39 | Enniskillen One A/Sgt and two A/Cpls made. |
| 3/12/39 | Major Grant DAQMG NID visited Regt. |
| 6/12/39 | Two WOs and one Sgt of SWB returned to LONDONDERRY. Four further purchased vehicles brought down from BELFAST for instructional purposes. |
| 9/12/39 | Strength 186 ORs |
| 11/12/39 | The Regt splits into two Sqns. HQ Sqn remains at Castle Bks, A Sqn takes over accommodation at COUNTY HALL. Transferred to A Sqn are:-
| Capt Lord O'Neill | Sqn Commander |
| 2Lt Coey | 2/ic |
| TSM Wells | A/SSM |
| 3 Sgts, 4 Cpls, 10 LCpls, 70 ORs. |
|
| 13/12/39 | 45 recruits enlisted. 20 posted to A Sqn. |
| 14/12/39 | 2Lt M Montgomery gazetted. Seven LCpls made A/Cpl.
|
| 16/12/39 | Appointments:-
| OC HQ Sqn | Capt Booth |
| Security officer | 2Lt Pomeroy |
| Sports officer | 2Lt Butler |
| Technical officer | 2Lt Adams |
A total of 8 NCOs have now completed a fortnights course at Victoria Bks. |
| 18/12/39 | CQMS Jordan posted to Regt. |
| 23/12/39 | Strength of ORs HQ Sqn 119, A Sqn 108. |
| 27/12/39 | Main guard will be 24 hours both at Castle Bks and County Hall. |
| 28/12/39 | Capt Stronge granted sick leave. |
| 29/12/39 | RQMS Tanner posted from reserve 15/19th Hussars. Major the Earl of Erne proceeds on Tactical course at Tidworth. |
| 30/12/39 | Increasing numbers of cases of ORs "going sick" with "Tonsillitis". 'Gargling' parades introduced twice per day. |
| 31/12/39 | Strength 237 ORs. |