The shallow draft of the Viking longship enabled it to skim fast inland, whether into otherwise unnavigable estuaries; or to safety in its own Scandinavian fjords, see them at
The Sea Stallion, Viking Longship, Denmark Road Ways. Although the Irish monastic communities raided each other, and the Irish people also raided the Irish monasteries and had for about 100 years before the Vikings:
"Irish monasteries provided important economic and political focal points to the Vikings for provisions, precious goods, livestock, and captives. This was also the reason that monasteries were a favorite raiding spot for Irish leaders both before and after the arrival of the Vikings."
See http://www.rootsweb.ancestry.com/~irlkik/ihm/ire800.htm; see also http://www.deremilitari.org/resources/articles/fitzpatrick.htm
Long boats like the Sea Stallion was documented, we understand, as particularly favoring raids on monasteries like Glendalough, and Clonmacnois. It is some 30 meters in length, or about 98 feet - see http://www.simetric.co.uk/metres_to_feet.php/
This replica sailed into Wicklow in 2008, Wicklow a name of a town that itself is Viking in origin, as "Viking's Meadow" or "Viking's Beacon" , see http://www.wicklowsailingclub.com/Misc/sea_stallion_of_glendalough.htm/. See the video at http://www.youtube.com/watch?v=t4T2f6WG5yg
Viking raiders soon morphed into settlers, see http://www.wesleyjohnston.com/users/ireland/past/pre_norman_history/vikings.html
For us, at our earlier visit, Wicklow was the site of an old gaol, jail, from 1702, with only modest reforms easing the conditions, see it at http://www.wicklowshistoricgaol.com/history.htm/ Many prisoners were transported from there to Australia, or the Americas, and even at a profit to the operators.