Per "The Bretons", by Galliou and Jones, In the last half of the 9th century, the Vikings made sporadic raids into Brittany, but as they experienced determined resistance from CHARLES THE BALD and ROBERT THE STRONG, they turned their main energies towards England.
Stuart's "Royalty For Commoners" (405:38) says he was Count of Rennes 887-890 and that he defeated the Norsemen at Coeson in 890.