Complex sentence: On such raids the Vikings set up impermanent camps, which were called longphorts by the Irish--this period of Viking raids on the coasts of Ireland has been named the longphort phase after these types of settlements.
Split into simpler sentences: On such raids the Vikings set up impermanent camps, which were called longphorts by the Irish. This period of Viking raids on the coasts of Ireland has been named the longphort phase after these particular types of settlements.