Trim Castle - County Meath
While it is still impressive today, it must have been nothing short of awesome in its heyday - Trim castle, guarding the Boyne crossing, was the main stronghold of the Anglo-Norman de Lacys. These days it dominates the heritage town, a walk around its perimeter will help you to stretch your legs.
Get more details in my guide review of Trim and Trim Castle.
Plantation Castles of County Fermanagh
On the shores of Lower Lough Erne you will find several fine ruins of "plantation castles", fortified manor houses. These were erected by Protestant-Scottish settlers to defend their domain from marauding Irish natives. Who slightly disagreed with them staying. The ruinous state of the castles often reflects a succession of gate-crashing objectors.
Dunguaire Castle - County Galway
Location, location, location ... the builders of this castle certainly made sure that attackers did not have an easy entrance. By placing it on a rocky headland. Which also ensured its popularity on holiday snaps and a few thousand postcards. These days the castle is a museum and (in the evenings) offers "medieval banquets". As does the slightly less spectacularly sited Bunratty Castle.
Visit the website of Dunguaire Castle for more information.
Ashtown Castle - Phoenix Park, Dublin
If you want to see a real tower house, the typical Irish castle, you do not have to stray too far from Dublin's city center. Right next to the Phoenix Park Visitor Centre you will find Ashtown Castle. Pay a short visit for the obligatory photo or (better) visit the center and learn more about the history of Europe's largest municipal park.
Find out more about Ashtown Castle and the Phoenix Park Visitor Centre here.
Enniskillen Castle - County Fermanagh
Location was important here as well - the castle guards both the Erne ctossing as well as the connection between Lower and Upper Lough Erne. Today the picturesque "water gate" with its twin turrets is still one of the memorable sights of Northern Ireland. Like Athlone the castle has been rebuilt in modern times, the best views are from the lakeside (though the museum inside comes highly recommended).
Check out my guide review of Enniskillen Castle for more details.
If there was a price for rebuilding, Dublin's castle would be a strong contender. Almost nothing of the medieval fortress near the black pool (dubh linn in Irish) has been left untouched. To the extent that the oldest-looking parts are actually quite late additions. Also enjoy the Dubh Linn Gardens and the unmissable Chester Beatty Library while you are there.
Donegal Castle - County Donegal
Another tower house - but long converted into something resembling an small palace or at least a and manor house. Situated in the heart of Donegal Town, the castle has been altered beyond recognition. But as this happened a few hundred years ago, the rebuilt structure remains a historical building. Mind you ... it certainly looks bigger in pictures.
Visit the Donegal Castle website for further information.
Athlone Castle - County Westmeath
Athlone's main attraction (apart from the Shannon and shopping facilities), the castle has been rebuilt several times. While there is nothing "romantic" about it anymore, one can see why it was strategically important - you would have to defeat the garrison to cross the river. The museum in the castle illustrates this fact with a fine exhibition.
Find out more about Athlone Castle in my guide review.
Limerick Castle - Limerick City
One of Limerick's main attractions, the Anglo-Norman castle is commonly known as "King John's Castle". While the connection to John Lackland (the evil Prince John of the Robin Hood legends) is not too strong, the castle walls still are. Maybe the safest place to be in modern-day Limerick, also known as "Stab City".
For additional information, visit the website of King John's Castle.
Stormont Castle - Belfast
"Castle" is somewhat of a mis-nomer for this thoroughly 20th-century edifice, built to house the government of Northern Ireland after partition. Long unused for this role due to "the Troubles", it has been given a new lease of life during the peace process. And it cannot fail to impress, especially on a sunny day. Though the strategically placed statue of arch-Unionist Carson may spoil the view for some visitors.