 Hudson argues that Tryggvason 's return to Norway in 994 coincided with the temporary expulsion of Sigtrygg from Dublin by his rival Ivar of Waterford , . Ivar may have already ruled there from 989 until forced out by Sigtrygg in 993 . Much depends on the interpretation . Either way , Sigtrygg was back within a year . In 995 , he and his nephew , Muirchertach Ua Congalaich , attacked the church at Donaghpatrick in County Meath . In retaliation , Máel Sechnaill entered Dublin and took the ring of Thor and the sword of Carlus . Sigtrygg then attacked Kells and Clonard in 997 . In 998 , Máel Sechnaill and the King of Munster , Brian Boru , forced Sigtrygg to recognise their lordship by giving hostages . 
