Hello JLyons, and welcome to the forums, the game in indeed the hobby! 
I see you noted that the next Empire event will be April… plenty of time to prepare, check things, acquire any kit, and chat to new friends.
Minis, D&D, and re-enactment… sounds like an excellent portfolio to be honest.
As for how to get to the events from Northern Ireland… I’m not too sure there. If you can manage public transport to, say, Milton Keynes, I suspect there will be a shuttlebus service between the train station and the site. You’ll have to check the fine detail closer to the time.
Alternitively, you may find it helpful to ask about looking for a car share to some extent. Empire draws player from across the UK (and Europe, and beyond…), so you may find someone to travel with, or even someone willing to give you a lift in exchange for petrol money. Even if it’s just someone to meet at a station and go halves on a taxi. Things to investigate over the winter…
As for what to do, well, almost anything. You said you were looking at the Marches, and at Landskeeper. That archetype, like all of them, is similar to a pre-generated character for D&D: it gives ideas of what to be doing, attitudes to things, suggestions for roles, and some hints for effectively playing up to the archetype. And yes, you can certainly turn up as one of them from the start. Heck, as they are almost wandering druids (who will beat me with their sticks for saying so), turning up as a Landskeeper looks to be a perfectly reasonable reason to wander round poking things, asking questions, and being unknown to everyone. How many druids visit their nations capital normally? 
There are also a lot of threads asking (and I hope answering) that question on these forums; browse at will and feel free to ask questions.
For who you want to be… well, you may find it useful to play pretty much yourself at first, with one or two memorable character traits: an accent or verbal tic, yes? A strong opinion or predjudice (IC), or a habit of fiddling with something. I can see a landskeeper talking to their stick, or offering around nuts all the time, or frequently tasting the local grass… (cliches… ). If it turns out that your character and skills are utterly wrong for you, or you just want to tweak things, PD are very understanding and will allow you to re-roll your character during or after the event, if needed.
There IS a point where a new player could bite off more than they could chew. I advise against declaring your intention to sit the Throne on your first event. Empire IS a deep pool to jump into for your first LARP, but to continue the metaphor, it’s pretty calm and there are lifeguards watching.
The game will be starting it’s 8th year in (sigh) 2022. Seven years of complex politics, national struggles, marching to and fro of mighty armies, dread curses and heroic deeds, and enough stories to fill a shelf of epic fantasy novels…
YOU NEED TO KNOW LESS THAN 1% OF IT
In order to not make a fool of yourself, you will need to know:
Your characters name.
Their nation (read at least the front page of the national brief)
Which province they’re from in that nation.
Approximately what they can do in mechanical skills.
(That’s it. you could write it down on an A6 notecard and check it frequently. Which brings me to…)
Costume and kit and accessories is a whole 'nother subject, but I’d reccomend picking up a small (A6 notebook with a neutral cover, decorating it appropriately, and scribbling useful notes in it. Details of your character, what your skills do, things to investigate in play… it will quickly get filled with things like the names of friends (and enemies), things you’ve seen, questions you want to ask, and deals to keep track of. Loads of people use such notebooks, pulling it out to check a detail will not be remarked upon, and buy you a quiet moment as needed.
Speaking of which:
There are New Player Briefings before the game starts, on Thursday and Friday until Time In on Friday 18:00, workshops and meetings and explanations on all aspects of the game.
Your Egregores (national magical mascots) are there to help new (and old) players in navigating the game. They are on your side, and their purpose is to help you. Ask them for help, explanations, directions, minor quests…
“I’m new to Anvil” is a code phrase, meaning “Help I’m a new player and don’t entirely understand”.
There will be a LOT of new players next event. You are not alone. Whether raising your weapon on the battleline, your purse in the marketplace, or your voice in song, there will be new brothers/sisters/others to be confused but enthusiastic alongside you.
Now, I’ve rambled long enough myself, so I’ll shut up and let you think up some more questions.
As well as these forums, there are many Facebook pages, for nations and for interests (Artisans, Magicians, etc), on which you may find assistance. And of course there’s the wiki, don’t fall in. 
I would advise having a poke around, making some friends, perhaps finding an in-game group to join.
Here is the Facebook page for the Marches, for reference: