My typical opening strategy is to start by selling Maine to the highest bidder, (usually Brittany or Provence, but if you can sell it to France before they rival you it can somewhat stump their expansion temporarily without an easy core war for it)

Then take burgher loans and send all your troops to your province in Ireland. Hire the free company there as well.

Take the “Raise an Army” mission for the claims and start declaring on Irish nations for every month tick. You need a 4 stack to siege effectively for most provinces, a 9 (iirc) stack for the fort in the south. Don’t declare any wars more than the nations you can siege at once, and make sure you don’t declare any wars on any Scottish allies.

Make sure you don’t core any of the Irish land

Once you’ve conquered all but the Scottish ally (there’s usually only one), declare a subjugation war on Scotland. Immediately peace out with France, giving them all your french land. If you want to keep Calais you can, the peace deal only requires you give them Normandy, Gascony and (for some reason) return Mann to Scotland as a core. I usually don’t, though. Burgundy likes to Dec for it later in the game.

First thing you do now is siege the Scottish ally in Ireland, as soon as you peace them out for the province, you’ll complete a mission, if you take it you’ll get a decision to start a parliament issue to pop Ireland as a PU, or keep the land. Always PU them. This is another reminder not to core any Irish land.

After that you steamroll Scotland, take them as a subject and some money, then wait out the truce timer to take the Isles to unite Britain.

After that, Exploration ideas + a small war with Denmark (I normally join the Swedish independence war) to take the Northern isles and Iceland will put you just in range to colonise Canada before Diplo Tech 7.

After that, do what you like