The Angevin Empire is a modern term applied retrospectively to the lands of Henry II of England, consisting of at their largest extent, the Kingdom of England and duchies or counties of Normandy, Anjou, Poitou, Maine, Gascony, Touraine, Béarn and Aquitaine; with Brittany, Wales, Toulouse, and Ireland held in vassalage to some degree. The lands of Normandy and England had been in personal union since the time of the Norman Conquest. The prospect of a union between the lands of Anjou and Normandy was first suggested during the time of Henry I, who intended his daughter, the dowager Empress Matilda (Maud) to succeed him as Queen. A dynastic alliance with the counts of Anjou made a lot of sense to Henry. The counts of Anjou had recently gained control of Maine though the marriage of Fulk V to Eremburge of La Flèche, and their increased power and influence in Northern France offered a potential threat to Henry's control over Normandy.
Unfortunately for Geoffrey and Matilda, this plan failed as the English Barons ignored the wishes of Henry, choosing Stephen of Blois as the next king. This led to a time of troubles known as The Anarchy in the Anglo-Norman nation as Matilda and Stephen fought for control, ending with the signing of the Treaty of Westminster which allowed Stephen to remain king, but ensuring Henry, the son of Geoffrey and Matilda, would succeed him. The lands under direct control of the Empire were essentially complete with the marriage of Henry II to Eleanor of Aquitaine. Over the next two decades Henry consolidated his power, taking surrounding states, most notably the duchy of Brittany, into various forms of vassalage.
The height of the Angevin Empire came during the reign of Richard I of England, an avid imperialist and unquestionable patriot of Christendom. Richard wed Berengaria of Navarre, to provide a strong ally to the south of his realms, possibly with the hopes of inheriting the Kingdom of Navarre as part of her dowry. This was in like kind to his mother's Aquitaine being held by Henry and added to the English Crown. Richard's marital negotiations for his nephew Arthur of Brittany, were to add Sicily into the English Domain. If the Third Crusade had met with success, Richard would have come back to regain the Kingdom of Cyprus and possibly even the Kingdom of Jerusalem (his great grandfather Fulk, was King of Jerusalem).
The reign of John saw the end of the Angevin Empire. John can claim to be the first English king (as opposed to a king of England) as he was born in the lands, and spent much of his time within England. Contrary to received impressions of history, in the early part of his reign John was popular among his barons. However his political talents were no match for the successful expansionist strategies of Philip Augustus, and the French lands, with the exception of Gascony and the Channel Isles, were lost by 1202. The true end of the Empire could be seen in 1216 when the Barons of England offered the crown to Prince Louis (future Louis VIII of France), thus confirming the supremacy of France. However ideas of reclaiming the Angevin lands in France remained a dream of English kings, and was a contributing factor to the Hundred Years' War.
The first of these Angevin dynasties ruled England in some form or another from the reign of Henry II, beginning in 1154, until the House of Tudor came to power when Richard III fell at the Battle of Bosworth Field in 1485. The name "Plantagenet" is derived from the broom flower (planta genesta). It originated with Geoffrey of Anjou, father of King Henry II of England, because he adopted the flower as his emblem, often wearing a sprig of it.
Henry II invaded Ireland in 1171. As a result of the Treaty of Windsor in 1175 between Henry II of England and Rory, High King of Ireland, Ireland became subject to the English Crown. English kings ruled through the appointment, first of Justicars and later of Lords Deputy and Lords Lieutenants. In 1177, Henry II created his son John, "Dominus Hiberniae" Lord of Ireland, at the same time two Norman's, Robert Fitzstephen and Milo de Cogan were granted the Kingdom of Cork between them. Philip de Braose was granted from Thomond, the Kingdom of Limerick. Later in history, the more famous and much married Henry VIII was the first to use the title of King of Ireland.