Drogheda (/ˈdrɒhədə, ˈdrɔːdə/ DRO-həd-ə, DRAW-də; Irish: Droichead Átha [ˈd̪ˠɾˠɛhəd̪ˠ ˈaːhə] ^ⓘ, meaning "bridge at the ford") is an industrial and port town in County Louth on the east coast of Ireland, 43 km (27 mi) north of Dublin. It is located on the Dublin–Belfast corridor on the east coast of Ireland, mostly in County Louth but with the south fringes of the town in County Meath, 40 km (25 mi) north of Dublin city centre. Drogheda had a population of 44,135 inhabitants in 2022, making it the eleventh largest settlement by population in all of Ireland, and the largest town in the Republic of Ireland, by both population and area. It is the second largest in County Louth with 35,990 and sixth largest in County Meath with 8,145. It is the last bridging point on the River Boyne before it enters the Irish Sea. The UNESCO World Heritage Site of Newgrange is located 8 km (5.0 mi) west of the town.

Drogheda was founded as two separately administered towns in two different territories: Drogheda-in-Meath (i.e. the Lordship and Liberty of Meath, from which a charter was granted in 1194) and Drogheda-in-Oriel (or 'Uriel', as County Louth was then known). The division came from the twelfth-century boundary between two Irish kingdoms, colonised by different Norman interests, just as the River Boyne continues to divide the town between the dioceses of Armagh and Meath. In 1412, these two towns were united, and Drogheda became a county corporate, styled as "the County of the Town of Drogheda". Drogheda continued as a county borough until the establishment of county councils under the Local Government (Ireland) Act 1898, which saw all of Drogheda, including a large area south of the Boyne, become part of an extended County Louth. With the passing of the County of Louth and Borough of Drogheda (Boundaries) Provisional Order 1976, County Louth again grew larger at the expense of County Meath. The boundary was further altered in 1994 by the Local Government (Boundaries) (Town Elections) Regulations 1994. The 2007–2013 Meath County Development Plan recognises the Meath environs of Drogheda as a primary growth centre on a par with Navan.

The town is situated in an area which contains a number of archaeological monuments dating from the Neolithic period onwards, of which the large passage tombs of Newgrange, Knowth, and Dowth are probably the best known. The density of archaeological sites of the prehistoric and early Christian periods uncovered in the course of ongoing developments, (including during construction of the Northern Motorway or 'Drogheda Bypass'), has shown that the hinterland of Drogheda has been a settled landscape for millennia.

Despite local tradition linking Millmount to Amergin Glúingel, in his 1978 study of the history and archaeology of the town, John Bradley stated that "neither the documentary nor the archaeological evidence indicates that there was any settlement at the town prior to the coming of the Normans". The results of a number of often large-scale excavations carried out within the area of the medieval town appear to confirm this statement.

One of the earliest structures in the town is the motte-and-bailey castle, now known as Millmount Fort, which overlooks the town from a bluff on the south bank of the Boyne and which was probably erected by the Norman Lord of Meath, Hugh de Lacy, sometime before 1186. The wall on the east side of Rosemary Lane, a back-lane which runs from St. Laurence Street towards the Augustinian Church, is the oldest stone structure in Drogheda. It was completed in 1234 as the west wall of the first castle guarding access to the northern crossing point of the Boyne. A later castle, circa 1600, called Laundy's Castle stood at the junction of West Street and Peter's Street. On Meathside, the Castle of Drogheda or The Castle of Comfort was a tower house castle on the south side of the Bull Ring. It served as a prison, and as a sitting of the Irish parliament in 1494. The earliest known town charter is that granted to Drogheda-in-Meath by Walter de Lacy in 1194. In the 1600s, the name of the town was also spelled "Tredagh" in keeping with the common pronunciation, as documented by Gerard Boate in his work Irelands' Natural History. In c. 1655 it was spelled "Droghedagh" on a map by William Farriland.

Drogheda was an important walled town in the English Pale in the medieval period. It frequently hosted meetings of the Irish Parliament at that time. According to R.J. Mitchell in John Tiptoft, Earl of Worcester, in a spill-over from the War of the Roses the Earl of Desmond and his two youngest sons (still children) were executed there on Valentine's Day 1468 on orders of the Earl of Worcester, the Lord Deputy of Ireland. It later came to light (for example in Robert Fabyan's The New Chronicles of England and France), that Elizabeth Woodville, the queen consort, was implicated in the orders given. The parliament was moved to the town in 1494 and passed Poynings' Law, the most significant legislation in Irish history, a year later. This effectively subordinated the Irish Parliament's legislative powers to the King and his English Council.

The town was besieged twice during the Irish Confederate Wars.

In the second siege of Drogheda, an assault was made on the town from the south, the tall walls breached, and the town was taken by Oliver Cromwell on 11 September 1649, as part of the Cromwellian conquest of Ireland and it was the site of a massacre of the Royalist defenders. In Cromwell's own words after the siege of Drogheda, "When they submitted, their officers were knocked on the head, and every tenth man of the soldiers killed and the rest shipped to Barbados."