How to get a second kingdom?
There are several different way of doing it.
I am assuming that the actual kingdom of Brittany title hasn't been created as of yet. Because if it already exists then you would just have to usurp it from the current owner through most of the same steps.
1) find a weak military Duke that own a title within De Jure Brittany. Start fabricate a claim on that Duke or alternative arrange a marriage where the title would eventually end up under your control (longer way but possible quicker with enough murder).
2) Repeat step 1 until you have at least 3 or 4 titles within De Jure of Brittany (in the rare case it isn't the normal 6 country due to De Jure changing then you need 2/3 of the titles).
3) If there is a King already for Brittany. Then usurp his title so you become the De Jure King. If it doesn't exist. Then you need to create it.
4) Make sure your newly acquired Kingdom of Brittany has law of succession to your liking. Otherwise it may fall to someone beyond your control soonly afterward if you are not careful especially if it is elective type succession.
You can try to get your heir to inherit both titles. You can do that by pressing your spouse's claim on England.
This method works if both Ireland and Brittany have some hereditary succession law, which they should have by default. It also won't work if England has a pure-agnatic succession and you are a man.
- Marry someone with a claim to the Kingdom of Brittany. This should be someone who does not give you a non-aggression pact with the king of Brittany (unless he is likely to die soon). You can find claimants by clicking on the "Claimants" button of the title window. Keep in mind, though, that weak claims can only be pressed against regencies, so get someone with a strong claim if possible.
- Have a heir with your spouse. If your spouse already has sons from an earlier marriage, they need to die.
- Press your spouse's claim on Brittany in a war. Alternatively, if the claim can be inherited, wait until your spouse dies and press the claim as/for your heir (this might be necessary in some cases when you married a woman).
- Win that war
Your spouse will become Queen/King of Brittany, which will make them an independent for now. But your heir will inherit both kingdoms when you two die. Steps 3 and 4 can of course be skipped if you manage to marry the actual heir to the English throne, but that's unlikely.