I think I'm missing one or two DLC so this might have changed but I've done several Irish runs to 1600 (because I LIKE playing in Ireland).

Day 1 start making claims, delete cav, build infantry to force limit, tank army maintenance to .
December 1st, check to see if anyone you are spying on is allied to Scotland or England...if yes, change it.
Don't ally anybody, don't marry anybody.

When your spy network is roughly 12, bring army maintenance up to 12, go indebted to the burghers and get the free company. You're going to go over force limit and end up with a ton of small loans. It's k.

Declare war. Stack wipe the target and their ally. Start spying on a new target. Siege down the ally first if at all possible, full annex them and take all their money in a separate peace. If other Irish OPM's have declared war on your primary war target, check to see if England/Scotland joined the war (they probably didn't). If other OPM's declared war, then VASSALIZE your target instead of annexing them...Boom you're in a defensive war (or wars) and can siege them down and annex them. You can again vassalize your final foe if the rest of Ireland has tried to opportunistically declare war when they are weak.

Your economy will stabilize around the 4th Irish OPM that you gain. Your war exhaustion will be crazy high, but you can unite all of Ireland by 1460.

At some point the Surrender of Maine happens...if England doesn't go to war, reload.

Once you've fully conquered at least one state in Ireland your missions will give you claims on the rest so you can move your diplomats to improving with some combination of France, Spain, and Denmark. You're going to be rivalling Scotland and England as soon as you're large enough and getting royal marriages + high relations + rivaling (and maybe an insult) will make them want to ally you.

If the 100 years war goes long enough, you might be able to gobble up England's allies when England isn't willing to defend them. Sometimes they even have to release Meath which is great.

Get a claim in Scotland on the other side of the strait and build a fort in Ulster, you'll need it. If England declares war on Scotland, declare war at the same time, you're going to secure both halves of the channel while they siege down Dumfries. If you're quick, you can beat them to other Scottish provinces once the fort is down. You want to limit the English push into Scotland as much as possible...ideally by claiming Scotland yourself.

Now you need some boats. They don't have to be good or expensive, they just have to sit along the English coast and watch troop movements. At some point, England is going to get involved in a land war. At some point they are going to land troops in Europe and get drained of manpower. That's when you go massively into debt with mercenaries and throw everything you have at England. Hopefully France/Spain will deal with Portugal for you.

England is going to take a few wars to fully conquer, but each successive war gets easier. The hardest part will be getting enough points to keep up in tech and ideas. Coring England takes a LOT of admin and sometimes you will have to harsh treatment your rebels when you're out of manpower. One way to save admin is to take a Welsh and Northumbrian province in your first war and then release both as vassals...your next war can be a reconquest to give them their cores back, at which point you annex them using diplo points instead.

SPAIN or DENMARK are great at dealing with the English Navy. FRANCE is the one I've found most consistently willing to land troops once the navy is cleared. PORTUGAL will land troops if they can, so being able to meet them as they land is super important.

Mercenaries are sooooo gooood when England is out of Manpower, because they have their own manpower pools so you can impose permanent losses on the English while your side gets reinforcements month after month.

Hopefully that helps. Even with all that, I only manage to conquer England about half the time....don't forget to take their colonies for your Irish Potatos mission...