Microlite... every-one is presuming the powered hang-glider type, or 'wight shift' microlite, with open gondola cock-pit; but it could just as easily be a 'fixed wing' or rigid, like a conventional light air-craft... just lighter.
I have a little experience of both; my Uncles a Microlitist; actually had to get the bit of better mown grass down the middle of the back field at the family farm registered as a private air-strip becouse he used it too much.... anyway, he has had both types, and I have to say I did prefer his old Pegasus weight-shift.
Helmet? Yes. You will have to wear one in an open cock-pit, I believe its private-pilot rules. Closed cock-pit, I dont think you do. Though this may be the open face type, which is easier to lift a camera to, but you will probably have a boom microphone in'front of your face so you can talk to the pilot. Its a LITTLE bit hard to shout over the roar of the engine, and propeller... wind isn't actualy that bad, these things are the aeronautical equivilent of a moped; their stall speed is something stupid low, like about 10mph, before they fall out the sky, and they only have to get up to about 15-20 before they'll start going up... And they go up QUICK mate. Very quick.
Think about your lunch before you go up. It can be a bit roller-coasterie... they have little 6" wheels on the ground and bounce along the bumpy grass strip, every tussock tipping the thing a wheel height... they dont have suspension! The frame is just a bit bendy! And you rush along the floor, almost laid down in a rally seat, your bum 3" off the floor.... (if they haven;t mown teh grass for a week it CAN be tickling your fancy!) like your in a go-kart.. then... then.... WhoW! wing goes back, and you're in the air, and climbing. Stays a bit bumpy till you are a couple of hundred feet off the deck, then smooths out as the ground effects dissapear, but when you do get a bit of a jolt, tends to be a strong one.
You will be flying visual-flight-rules, in uncontrolled air-space, which means pilot is not guided or under air-traffic control instruction, he has to look where he's going and make sure he dont bump into anything, but he is also restricted WHERE he may fly.
I never did my Private-Pilots-Licence; which I suppose is a bit of a wsted oportunity given the kite was just sat there and I would only have had to pay for the petrol and do a bit of maintenence to earn my hours, so I'm a bit hazy exactly what they rules are.... BUT if you want to get the pilot to get you over specific land-marks, be worth checking them out.
From memory, they cannot fly in controlled air-space, without permission from air-traffic control, and then only to cross' a corridoor unless they have filed a flight-plan to be on the coridor traffic schedules. See you are in Northern Ireland.... They cannot fly in military keep-out zones.... these may be extensive! And there are certain regs restricting flights around Heritage sites, and built up areas, that might put a spanner in the works regarding your plan to photo your house and National trust property.
These maybe worth checking; before you go up, might even be worth talking to the pilot and plotting a flight plan to get the sites you want/can.
Back to the job... its cold up there. You are likely to get given a flight suit to keep the chills out. These are cumbersome, and often dont have big or convenient pockets. What pockets they do have are designed around pilot or navigation tasks, like radio operation & map reading, not carrying cameras, and pilot I'm sure would apreciate you packing light.
I have gone up with cameras; and even swapped lenses in flight.. but its probably NOT adviseable! You really dont want to drop one at five thousand feet, and there's not a lot of floor to catch it! And you'll have cold hands and no-where to hang your gadget bag or anything. As Nod says, a Bridge maybe the better choice.
You wont be going up very high in a microlite, so for direct shots below, you wont need a lot of reach, but do beware the mechanics around you. Could only find a couple of aerial shots I took from the Peg, and they aint great... think I messed up process, but gives you an idea.
Framing higher angle overheads can be tricky; those nice pictures that come selling door to door, were usualy taken with medium format cameras, frame mounted, so you struggle hand holding over the edge! Lower angles are easier; and I think work better.
As with all photo's getting the framing right and interest in the shot is the important bit, only thing to worry about really in a microlite is higher shutter speeds against the vibration & movement.
Other thing with a weight shift microlite, is that the carriage doesn't tilt much.. well it can, but its a pendulum under the wing; it doesn't tilt like a motorbike or rigid wing when you go round corners; you want to turn left, pilot pushes the control bar to the right, that DOESN'T tilt the wing to the left, it tilts you in the pendulum the oposite way... then gravity tries to pull you back to centre and that tilts the wing to make you turn....
To get good shots from a microlite, I think most useful thing to do is your research on what you want/can photo, and planning where to put the flying tent (Their construction has more in common with one of them than the space shuttle, for sure!) to get the pics you want, rather than worrying about the actual kit.
But dont have high hopes of your shots; I'd go up primerily to enjoy the ride and take the bar.... any pics you get just bonus.... probably best momentos will be pics taken by your family on teh ground of you getting togged up, and getting in, and that one, if some-one can get down on the strip and get some snaps of the bird going up with you in it.... then of you staggering out at the end!