In my current game, as the small count of Leinster I started by creating the duchy, conquering some land with my ally Dublin, starting under my son, which of course became me when my count died. Well, from the two Duchies of Leinster and Meath I began to unite Ireland and worked to upgrade my holdings as quickly as possible. By the time of the first crusade for Jerusalem (before that there were failed crusades for Sicily, which had been taken over by the Muslims - didn't take part in those) I had the means to participate and somehow ended up as the main contributor, gaining my kingdom title... One that I didn't even aim for at that point. Well, just shortly after I was ready to create the kingdom of Ireland, which I did but at that point I had no wish to take care of two kingdoms so far apart and since I wanted to play Ireland I decided to give the kingdom of Jerusalem to an uncle who really liked me (which also gave me the opportunity to gt rid of two inconvenient brothers by setting them up as his vassals).
So here I was again, little king of little Ireland, with my dynastic ally in Jerusalem... slowly expanding through the island, rather peacefully because by joining some wars on the right side I made sure the political landscape remained fractured and all these one-county-counts were happy to join the expanding Irish kingdom. I had to fight a war against Scotland to push them out of the northeastern corner of my island but since they were occupied elsehwere even that went smoothly. So, there I was, united Ireland, what next? The decision was taken from me quickly by a call to arms from my uncle in the Holy Land (I think it wasn't actually my uncle still reigning... I don't remember exactly though). I set sail to help him hold off a Holy War on Sinai (which he'd aquired on his own) and a Jihad for Jerusalem. Easy... so I thought... Various other Christian powers tried to help as well but after about a decade of fighting the doomstacks my king died in the Holy Land leaving the fate of the kingdom to his son who had nothing better to do than to avenge his father and carry the cross as defender of the faith as well.. bla bla bla... During his seven-year-reign Patrick I never saw the beautiful meadows of his home, spending them all in the levantine desert until he followed his father by death in glorious battle.
Well, after that his underage son took over and I decided it might be the right time to leave uncle Willy to his fate and retreated home where I had to deal with a nice little succession crisis

- Still the youngling turned out rather well and became a strong king of Ireland, forfeiting overseas adventures for a slow expansion into the lands of our Scottish neighbours. He did participate in a crusade, mainly to get the trait and some prestige. Jerusalem (which was lost after I pulled out, my families branch returning home) was retaken and handed to England. My plan was to take over Scotland and Wales and from there slowly build up to take up arms against England and create the Empire of Brittania.
What I didn't expect is the marriage policies of my ancestors paying off. Through some kind of connection with the english throne and a crisis on their part, my king suddenly ended up with a succession CB for the crown of England and Jerusalem (the funny thing being that Jerusalem was a landless title. The English had lost all the land to the Muslims again but apparently none of the Sheikhs bothered to usurp the kingdom)

! Well, my king really wanted those to complement the Irish gold on his head and who was I to refuse? It wasn't a big risk anyway, all I could really lose was the favor of some underage english king who probably wasn't gonna last very long anyway. So I declared war and raised my levies and suddenly THREE english dukes offered to swear fealty to me. I have no idea how that worked, shouldn't they have to be independent for that? Anyway, of course I accepted and even though it was tight, I won the war and my king found himself king of Ireland, England and Jerusalem (still landless title).
Now young Theobald (formerly known as King Theobald I of England) was still the biggest landowner in my newly aquired realm and that was... not so good, because he didn't really like me. Then I had a bright idea. I built holdings in his counties where I could afford it and turned over the newly created vassals to him. The relationship bonus edged him over the threshhold to accept my offer to become his Guardian and bam! No more problems on this side. I just had to take one more county in Wales, which was quickly done over a ducal de jure claim and was ready to create the kingdom of Wales and the Empire of Britannia. Made my new fosterling king of England again and turned the kingdom of Wales over to a nice relative. Awesome! Little king Theobald turned Irish under my tutelage. Even more awesome!
A little later the Pope called another crusade for Jerusalem, the third if I counted correctly and the best thing was, I wouldn't even need to participate. Still holding the crown they would only waste their soldiers to win the Holy Land for me! I couldn't resist though, seeing that I had so much more manpower now I joined the run and we started fighting for what was rightfully mine... Well... another king dead in the sands of the Negev but the crusade was eventually won and the kingdom turned over to a loyal, Irish vassal.
So here I am, Scotland, which was my long term goal since... I don't know... was still not in my hands but instead I have England, Wales, Ireland and Jerusalem and I'd say the Highlanders are just a matter of time... Turned out differently and better than I thought... one of my favourite games so far