Doing so will also give you plenty of time to reduce your threat level, avoiding other third-parties from getting involved.
In my opinion, taking out Ireland first is easier, for this reason you can target small duchies and walk right over them as opposed to a full-on invasion of Scotland. Now you're left with Scotland, which is likely still in one piece, and Ireland, which is almost definitely divided into small independent rulers. This will also give you the bridge to enter Ireland. Since Wales is still split between Powys and Deheubarth, this should make it ever easier, as what little men Wales has is now split in half. From there, I would recommend first annexing Wales, either by making false claims to the land or by other means. Assuming, like me, that you don't have any DLC of any kind which could potentially make this a faster ordeal, you are gonna want to start as William the Conqueror (specifically the one after he has conquered England.