Both are good choices, though my choice ideal is the 800s, because as Some Bloke put it, a Norse/Irish conquest of England would be nice. Turgesius/Thorgest is my favorite choice.
Well Imar was in the 800s; his reign started in 857 to be exact. I'm not sure its really possible to get too much further back then that - the Kingdom of Dublin itself was established around this same time, and that marks the time when the Norse really came to settle in Ireland. Prior to that time, most Norse interactions with the island were going to be raiding parties and traders; there would have been some small communities forming, but nothing too substantial. If you are going to get a Norse Ard Ri, they are going to need a base of support, and I'm not sure you can really do that prior to the 850s or so.
As as for the bit about a Finngall or Dubhgall Irish ruler who invades England ... There is a good likelyhood that Imar was part of the Great Heathen Army and would have been Ivar the Boneless. So, he came pretty close to doing that anyway

Probably your best bet