Largest Norman castle in Europe...Braveheart was filmed here. The keep is open for tours.
If you don't want to drive too far or have limited days, the part of Ireland you will be in is marketed as the Ancient East, which it is...particularly cool spots close to Trim that you will see on this website...Slane Castle, Hill of Tara, Newgrange...the wedding will take place beside Loughcrew Cairns https://www.ireland.com/en-us/myireland/attraction/152cf3276e694d7da22a0abcee60d3a1/loughcrew-cairns/#
A nice spot to go for lunch and sample some Irish whiskey
A great way to see Dublin
This is a great afternoon if you just want to sit back, relax and see Dublin city