There's a regular poster here who used to have a whole tagline based around
"How do I trade?"
"How do I get to Ireland?"
"Why is the pope such a bar-steward"
Chorus: Viking_mobilise.wav
...so I'm guessing you're the 5,437th person to come here and ask this. No matter.
You will need the following:-
1 castle up to keep level
1 port in the (coastal) province where this keep is
1 shipbuilder (4 yrs build)
'n' ships in a connected string of sea areas from your port to any sea area adjoining Ireland (3 yrs to build each ship)
0 ships owned by enemy factions blockading these sea areas (sink them, wait for them to move out of your way, eliminate the faction entirely, or obtain a ceasefire - if you can - to end the blockade)
1 army in the province with the port, ready to ship out and invade (pick up the stack: Ireland should light up indicating a valid move, drop the stack on Ireland to do battle).
Do NOT take your king on this expedition. I did, successfully, but others report mouy mal things happening afterwards. Search for threads on factors affecting province loyalty and/or causes of Civil War.
You cannot get agents into Ireland to assess the size of the rebel army there unless they go whacko and build a port (normally they're too skint). I don't wish to spoil the surprise by telling you the size of the army you're up against, nor the troop types.
I will, however, warn you that although you can beach-land troops without a port to receive them, some twist of logic dictates they cannot get back into the same beached ships they climbed out of. They will not be able to return to the mainland until you have 1) won the battle, 2) completed the siege 3) built another port. Make sure your garrison size back home will be up to scratch after they depart for the invasion.
Needless to say, the absence of a path of retreat means that the army will be lost in its entirety if you lose the battle. I won, so I don't know what the Irish policy on return of prisoners is but ransoming may get some to rematerialise back home, should the worst happen.
Hope this helps. Good luck.
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