Three slight addendums to that, the Statutory Instrument 21/2008, not 2008 Firearms Act(thankfully the most recent of those was 1964 IIRC.

The 24 inches barrel is equal to or greater, so you can have a 24 inch shotgun barrel as non restricted.

Also 410 isn't exactly uncommon, a lot of country gun dealers do more ammo sales in that than 28 gauge for example. Since 410 is very popular as a garden/small game gun vs target or clay it doesn't get seen very often mind you.

410 pump is somewhat available in the country, lever action I've yet to see a 410 here, but they can certainly be gotten.

Adler Arms(Turkish Company) makes a 410 lever action which is reasonably well regarded in Canada where they are imported to:

https://adlerarms.com/en/adler-a-410-lever-action-shotgun/?id=265

Henry also makes one in 24" with a side loading gate which is very nice:

https://www.henryusa.com/shotgun/lever-action-410-shotgun-side-gate/

As far as pump 410s go Mossberg makes some, even a suppressed one is in McBrides currently:

https://mcbrides.ie/products/mossberg-500-hushpower

I know other manufacturers do make pump 410s too, particularly Turkish ones, but might be best to pop into your local gun shop and see what they might be able to source.

I can also recommend http://www.fingalsports.com/ & Patrick O'Mahoney if you are looking for the Henry lever action in particular, as they deal in a lot of classic firearms, Paddy in lever actions particularly so would likely be able to get a Henry lever action in.

Failing all that have a look on the Mossberg(for a pump) or Henry USA/Adler Arms websites for distributors/dealers in Ireland and contact them as, being agents, they should be able to order in any of those manufacturer's firearms & accessories.

No worries, I have a soft spot for 410s having been the first shotgun that I shot ;-)

The Adler Arms is pretty nice, especially if you prefer the underside loading a la pump actions have, but personally kings gate(side gate) is just sweet :-D

Oh Mossberg shotguns are very good, popular, relatively cheap(their Maverick 88 series in particular, but only available in 12g and 20g), and a lefty friendly safety mounted on the rear of the top of the receiver.

Ah that Marlin 1895 is pretty damn sweet, shame about the 22" barrel making it restricted(for all the difference 2 inches makes 🙄).

I'd say for cool factor go lever action, for cheapness and efficiency pump wins, but I think you and himself will be pretty happy with either ;-)

I'm sure other manufacturers would have some models but Hatsan springs to mind:

https://hatsan.com.tr/product/escort-ps/

https://hatsan.com.tr/product/escort-xtreme/

Both available in 24" barrels.

Not a fan of the paintjob but there is a sweet Benelli M2 in 20 gauge too, albeit with a 26" barrel:

https://www.sportsdengunroom.ie/products/benelli-m2-camo-20g-4867

I think Armsan also do 20 gauge semis:

https://armsan.com/en/products/

I believe Dom's was a dealer for them a while back, could be mistaken but worth giving a buzz and asking:

https://domscustomrifles.ie/i/shotguns

Or email Armsan themselves and see who is the distributor for Ireland/UK, and from there get the gun dealers that distro sells to, for the long way round as it were 😉