Hello there
My favourite faction is Ireland, and I find that the best strategy is to:
A) Kick England out of your island immeadiatly - use spys and assassins to make them rebel also
B) Let your economy grow - No one EVER attacks the island, and all your objective provinces are there.
C) Get a couple of decent sized stacks, and head to Launston , and keep working your way east until you have all of southern most England.
D) By now, England will usually be depleting rapidly, so make an alliance with whoever is at war with them - Wales is usually Ideal
E) Once youve delt with England, rid yourself of the Welsh. This way you will have half of the mainland and Ireland.
F) Work your way north to Scotland and eventually the Norweigans if theyre still there.
This strategy varies once you go to the mainland because unless your using toggle_fow you wont know whos got the biggest foothold.
Remember, if your insurtion on the mainland ever backfires, just retreat to Ireland, and youll be safe there. In my experience, Ireland itself is very very rarely attacked.
Hope this helps
XKillerX