Both Gaill and Gaedhil, so dissimilar in many ways, benefited by their intercourse with one another. In Ireland the Vikings played an important part in the development of trade; they also promoted the growth of town life. We may trace the beginnings of the seaport towns, Dublin, Limerick, Waterford and Wexford, to the forts built by them near the large harbours in the ninth and tenth centuries. In Dublin coins were minted for the first time in Ireland[80] during the reign of Sihtric Silken Beard (c. 989-1042). Moreover, the large number of loan-words from Old Norse which made their way into Irish shows that the Irish learned in many other ways from the invaders, notably in shipbuilding and navigation.
So far as literature and art are concerned, the period of the Viking occupation is one of the most interesting in the history of Ireland. In spite of the destruction of the monasteries and the departure of numbers of the monks[81] to the Continent the work of the great schools was carried on and there was considerable literary activity;[82] in 914 and 924, respectively, the great crosses at Clonmacnois and Monasterboice were set up; cumhdachs, or book-shrines of plated gold and silver, were made for the three great manuscripts, the _Book of Kells_, the _Book of Durrow_ and the _Book of Armagh_; carved gold, silver, and bronze work reached a high level of excellence in the famous Ardagh Chalice and the Tara Brooch; and during the years which intervened between the battles of Gleann Mama and Clontarf, Romanesque architecture was introduced into Ireland. Irish art did not remain wholly free from Scandinavian influence. In the Cross of Cong (A.D. 1123) the Celtic interlaced patterns are found side by side with the “worm-dragon” ornament, while the crosier of Clonmacnois, the psalter of Ricemarsh and the shrine of St. Patrick’s Bell are decorated in the style known as “Hiberno-Danish.”[83]
The Vikings, on the other hand, came under the influences of Irish art and literature. We find marks of Celtic influence not only in the sculptured crosses erected by the Norsemen in the North of England and Man, but even in Scandinavia itself.[84] Moreover, there are strong reasons for supposing that the rise of the prose saga among the Icelanders may be the outcome of their intercourse with the Irish in the ninth and tenth centuries.