Most of Howth backs onto the extensive grounds of Howth Castle, built in 1564 but much changed over the years, most recently in 1910 when Sir Edwin Lutyens gave it a modernist makeover. Today the castle is divided into four very posh and private residences (the grounds are open to the public). The castle gardens are worth visiting, as they're noted for their rhododendrons (which bloom in May and June), azaleas and a long, 10m-high beech hedge planted in 1710.
The original estate was acquired in 1177 by the Norman noble Sir Almeric Tristram, who changed his surname to St Lawrence after winning a battle at the behest (or so he believed) of his favourite saint. The family has owned the land ever since, though the unbroken chain of male succession came to an end in 1909.
On the grounds are the ruins of the 16th-century Corr Castle and an ancient dolmen (a tomb chamber or portal tomb made of vertical stones topped by a huge capstone) known as Aideen's Grave. Legend has it that Aideen died of a broken heart after her husband was killed at the Battle of Gavra near Tara in AD 184, but the the dolmen is at least 300 years older than that.
Also within the grounds are the ruins of St Mary's Abbey, originally founded in 1042 by the Viking King Sitric, who also founded the original church on the site of Christ Church Cathedral. The abbey was amalgamated with the monastery on Ireland's Eye in 1235. Some parts of the ruins date from that time, but most are from the 15th and 16th centuries. The tomb of Christopher St Lawrence (Lord Howth), in the southeastern corner, dates from around 1470. See the caretaker or read the instructions on the gate for opening times.