Dunluce is a common photo opportunity for those visiting Giant’s Causeway and is a fabulously sited cliff top castle. The parking is just at the side of the road (I thing there is a small car park about 2k further west at Magheracross Viewing Point) and you get good exterior views and can appreciate the setting from the roadside. However it is well worth going inside as you can then appreciate the architectural detail that makes Dunluce Castle worth visiting.
Visiting Dunluce Castle
The castle has a small ticket office/shop and when were visiting we were the only other people at Dunluce. Looking online, it appears there is now a small car park at the castle. From memory either English Heritage or National Trust, or maybe both, members get in for free.
My Experience
We have visited many times for the views but the only time we have actually been inside was way back in 2011. I was working near Belfast for a few days so we took advantage and did a trip along the Causeway route including Dunluce, as detailed in my travel blog here.
The castle is in a fantastic setting on the cliffside, very similar to visiting Dunnotar in eastern Scotland. Part of the cliff has collapsed and the kitchens are now at the bottom of the Irish Sea. The exterior of the castle is the standard Scottish baronial style (reflecting the Scottish settlers transplanted to Ireland in the sixteenth century I guess) and the inside is very ornate with huge picture windows. Also on the site are the remains of the town of Dunluce which was destroyed in 1641 and appears quite revolutionary – grid plans for streets instead of the usual medieval warren and indoor toilets!
In 2021 I finally got to visit the Magheracross Viewing Point and look at Dunluce from that perspective. The viewing point is well laid out and gives a nice panorama from Dunluce to Portrush.
Dunluce Castle Photo Gallery
Magheracross Viewing Point 
Magheracross Viewing Point
History
Dunluce Castle was built in the 1200’s by the 2nd Earl of Ulster, Richard de Burgh, on the site of an earlier fort dating back to the Vikings.
In 1513 the castle was occupied by the MacQuillian family also known as Lord’s of the Route and later it passed to the MacDonnell clan. It was Somerled MacDonnell who improved the castle in a Scottish style in 1584 and when a ship from the Spanish Armada was wrecked on the rocks below the castle four years later, the MacDonnell’s sold the cargo and installed the cannon in the castle’s gatehouse.
The castle remained with the Mac Donnell’s until the end of the Battle of the Boyne in 1690 when it was left abandoned and fell into ruins.
References
Information on visiting Dunluce Castle is on the DiscoverNI site here.
The history above is copied from the Britain and Ireland castles website here.
The Wiki page here has more comprehensive information.