paragraph: The Vikings Arrive. Throughout this period, Ireland's political organization continued much as it had under pagan Celtic rule. There were still no towns; the site of Dublin was only a crossroads, known as Baile Atha Cliath ("City of the Hurdles," a designation still seen on buses). Communities clustered around monasteries and castles. From a.d. 795, Ireland was subject to repeated Viking raids. The Vikings sacked the great centers of learning for their treasures. In the ninth century they built a fort on the Liffey and founded Ireland's first town — Dubh Linn or "Black Pool." The remains of Viking fortifications can be seen today beneath Dublin Castle. The Vikings also introduced coinage and better ship-building techniques. In a.d. 988 the Irish kings finally united under the king of Munster, Brian Ború, and in a great battle drove the Vikings north of the Liffey. Their influence waned, and they began to be absorbed into the general population. The Irish now held Dublin and in 1038 the first Christ Church Cathedral was founded. Ireland Under the Normans In 1169 the Normans landed in Wexford, beginning the struggle between England and Ireland that was to dominate Irish history until independence. Norman incursion began with an internal power struggle. The king of Leinster invited Richard de Clare, known as "Strongbow," to come to Ireland to help him reclaim his kingdom (Strongbow's tomb can be seen in Christ Church Cathedral). The Norman invaders brought with them armor, the use of horses in battle, and the feudal system. Successive waves of Norman invaders followed Strongbow. 
question: After building a fort on the Liffey River, what did the Vikings introduce to Ireland?
answer: Coinage