I found Scotland pretty useless, so I conquered them myself + all the Irish minors. Then wait for massive revolts that wreck England, or stay in stalemate. Best ally to have IMO is Denmark, don't know why nobody said that, but as long as they have the kelmar union going you want the massive combined scandinavian fleet on your side. Denmark, Norway and Sweden's naval power combined with only one diplo slot is priceless. Especially Norway has some much needed heavy ships and allthough Sweden and Denmark have mostly galleys they're still usefull, toghether they have a few dozen ships. Also France, Spain and Portugal!! Portugal will probably ally England, so if you don't ally Portugal, change is they join the war against you. If you ally them and England attacks you they always join the defender instead of the attacker.
You'll probably be in a stalemate after that, if Denmark fails to keep the kelmar union over Sweden and Norway you might want to ally Burgundy, if you survive for a long time without anything happening the Dutch will eventually appear, they are probably the best allies you can have for naval warfare.
It also comes down to some luck. If you want to form Ireland, you'll need Meat, overall I've found it almost always relatively easy to split it off England during the war of roses. Support rebels and declare war on the right time. That strike should give you at least Meat. After that England does recover quickly... But with some good allies they will probably not attack you, so you can focus on colonization or expansion into Iceland / Scandinavia
It has been a while ago, several patches back that I played Ireland btw, I don't know if it's still easy to ally Portugal and Denmark, but I shouldn't know why itwould have changed. I believe they don't get the 'to many big allies' thing, but I don't know for sure if Denmark or Portugal are seen as big nations...
EDIT: I see a question about DOWing Scotland without getting France into the war as they gurantee. Normally Scotland almost always allies one Irish minor, so you can take them out by declaring war on an ally of them. Also if you declare directly while the hundreds years war is going on, they probably won't be able to land any troops as England is patrolling their coasts

Furthermore these days you could also make a March out of Scotland, perhaps it should produce more troops after that, but I normally don't use marches ^^