I would like to start occupying some provinces in Ireland, but my family has been marrying into Irish families for the last generation. As a result, I am allied with many people that I don't even remember allying with. Is there anywhere I can find a list of my allies?
The Diplomatic Relations map mode is probably your best bet. It's the scroll and quill button to the right of the mini-map. It shows the provinces controlled by your allies along with those controlled by your vassels and those to which your character has a territorial claim.
The Character Interface also has an Allies tab which will list any allied rulers. It's the far right-hand button in the row of buttons along the top.
Open your character page, at the bottom where you see your family, there are tabs for vassals and allies.
Listed under the allies tab are your your allies, and here is shown if they are 'callable' for war.
N.B: Although even if you ally is highlighted in green, you may not be able to call them under certain circumstances, such as if you already called them in the last 5 years or they are allied or have a truce with your intended target.