Just got off the phone with the DVTANI and there are indeed new emmissions testing in MOTs in Nothern Ireland.
There'a leaflet out detailing it, but (of course) it does not mention in any way Q plate or kit cars.
I rang them and talked to a tester who had no idea about what to do if presented with a Q plate car, and from him I got the phone numebr of one of the head guys in Belfast. I talked to him, he at first was adamant it would be tested according to it's date of registration. I pointed out that a Q plate's date of reg has nothjing to do with the age of it's engine, some cars are being built currently with crossflow engines from the 70s! Previously all Q plates had was a visible smoke check.
He agreed to look into it for me, and rang me back telling me that it's now to be done on the age of the engine (as in an SVA) with engines pre 1st August 1995 being exempt from the catalytic test. In this case you will need to bring proof of engine age.
It looks like the days of cat rental are over here in NI anyhow.
I guess I'm going to have to re-investigate the age of my engine to see if I can't prove it's dated before 1995.....
FYI Northern Ireland MOTs are done very differently to those across the water, they're performed at government test centre's not private garages. Unfortunately.

From the DVTANI Web Site


Exhaust Emission Test
This test applies to all cars, taxis, minibuses and ambulances with up to 12 passenger seats, and other vehicles up to and including 3500 kg design gross weight (DGW), which are spark ignition engined with four or more wheels.

Any of the following points will result in the vehicle being refused a certificate:

The engine is idling at a speed clearly above its normal idling speed.
The engine emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle.
The engine emits dense blue or clearly visible black smoke during acceleration, which would obscure the view of other road users.
The emissions cannot be measured because the tailpipe is damaged or an accessory is fitted which prevents insertion of the analyser probe.
The exhaust gas contains one or more gases in excess of the required limits for the associated test.
The required limits are as follows:
Vehicles first used prior to 1 August 1975 are exempt from a metered emission test.
Vehicles first used on or after 1 August 1975 and before 1 August 1986:
- Carbon monoxide (CO) 4.5% at idle
- Hydrocarbons (HC) 1200ppm (parts per million)
Vehicles first used on or after 1 August 1986 and before 1 August 1992:
- Carbon monoxide (CO) 3.5% at idle
- Hydrocarbons (HC) 1200ppm
Vehicles first used on or after 1 August 1992 are tested to the manufacturers specific limits (there are a few exceptions to this rule depending on the fuel and vehicle type).
While specific limits are too numerous to list the following is for guidance only:
- Hydrocarbons (HC) 200ppm
- Carbon monoxide (CO) 0.5% at idle
- Carbon monoxide (CO) 0.3% at fast idle
- Lambda: 0.97 - 1.03 Lambda (value of 1 for the optimum air/fuel ratio setting)
NOTE: It is important to remember the above figures can only be used as guidance as vehicle specific limits may be lower or higher.

Vehicles must be presented for test with the engine at normal working temperature. Therefore engine should be kept running on arrival at the test centre.