Hi Gary,
Yes, many record sets were destroyed during WWII.
[Including ones which had been moved out of London for safe keeping!]
The Royal North Downs were created (as the 24th Irish Militia Regiment) in 1800.
The Royal South Downs were created (as the 8th Irish Militia Regiment) at the same time.
There was a UK-wide reorganisation of Militia in 1833.
This resulted in the North Downs being designated as the 77th British Militia Regiment.
Its HQ was at Newtownards.
The Childers reforms of 1881 resulted in the creation of the Royal Irish Rifles.
This was the "County Regiment of Antrim, Down, Belfast & Louth".
Its HQ was at Victoria Barracks, Belfast.
The Royal North Down became the 3rd Battalion of this new regiment.
The Royal South Down became its 5th Battalion.
In May 1900 the RIRs 5th Battn (was RSDs), together with a 110 strong Company of Volunteers from the RIRs 3rd Battn (was RNDs), set sail for South Africa.
[132 did not return, and are commemorated by a statue in the grounds of Belfast City Hall.]
Sources:
https://www.royal-irish.com/stories/north-down-militiahttps://en.wikipedia.org/wiki/Royal_Ulster_Rifles----
Records of the RNDs extant at Kew are listed as covering the years 1800-1876.
[I expanded the search to cover other repositories known to them, with no result.]
So there appears to be an intriguing gap between 1876 and 1881.
Perhaps the "The loss of pre-1885 records as a result of fire ..." statement in the first source might explain it.
To resolve this you might consider trying:
a) contacting the Research Help Desk at Kew;
b) posting in the Rootschat "Armed Forces" specialist board;
c) contacting the Royal Ulster Rifles Museum in Belfast.
[The RIR was renamed as the RUR following partition in 1921.]
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If your GGF had previously enlisted with the RNDs, this would have had to happen before 1882.
So, at least 16 years previous to his (re-)enlisting with the RIRs.
Could this have been possible (based on his age)?
Your photo (in civilian clothing) provides no clue.
Militia did wear uniforms, but he may simply have been snapped off-duty in a "home life" setting.
Militia membership only required part-time attendance.
Many regiments were reduced to minimal (Cadre) strength during protracted periods of peace.
Men were let go from the payroll, but were encouraged to be available for recall when needed.
[Traditionally a Regiment consisted of two Battalions:
One on Reserve (at home, on Garrison duties);
The other Active (in the field, under The Colours).
Each Battalion consisted of 10 Companies, each (ideally) of 100 men.
Two of the Companies were designated as Depot
(sourcing resources, recruiting, teaching, training, administration, etc.) ]
There was no need to have served previously with a Militia regiment in order to sign up.
[However, such were targetted by the recruiting officers in the Depot companies.
They were given extra monies in their Bounty payments!]
Anyone could simply walk up to the Barracks and ask to be considered for enlistment.
Often several "pals" would join up at the same time.
If your GGF had had previous service prior to 1898, there should be some indication of this on his enlistment papers and/or the Paylist at the time he joined.
So, perhaps chasing the RIR records from 1898 should be your first port of call.
[Caveat: At times of mass recruitment/mobilisation, particularly in response to urgent international political goals, the standards of detailed records keeping were relaxed, sometimes very considerably, in the name of immediate efficiency ...]
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Hope that this helps you a bit.
Capt. Jock
ADDENDUM:
If he served in the RIR until at least 1920 his details should be available online within the UK's National Army Museum's collections. This includes the enlistment books of the five Irish regiments disbanded in 1922 (viz. The Connaught Rangers, the Leinster Regiment, the Royal Dublin Fusiliers, the Royal Irish Regiment and the Royal Munster Fusiliers):
https://www.nam.ac.uk/soldiers-records/personshttps://www.nam.ac.uk/explore/royal-irish-regiment