As a Heathen living in Belfast the prospect of Speedway over here would be extremely exciting, however I would be amazed if it were to happen.
The Belfast Giants Ice Hockey team have attracted good crowds as do the Ulster Rugby Union team, this shows that there is a market for spectator sports over here, but there really isn't a suitable venue. The dog track which services Belfast is at Ballyskeagh, which is nearer Lisburn than Belfast. I seem to remember it did hold a meeting once(someone with a copy of Homes of British Speedway may confirm) and there were meetings at other defunct venues in Belfast, but there has been no Speedway since early 1980's when Cradley rode a challenge match at Ballymena in front of a crowd of 5,000.
There is a great deal of interest in motorcycle racing, but it revolves mainly around the road racing tradition.
If someone was brave enough to have a go at introducing the sport I think they would probably get good crowds, but don't know if it would be enough to offset the transport costs of getting men and machines over the Irish Sea.
Don't know so much about the Dublin area, but again I suspect a GP round would be a risky but possible venture, but unlikely on a regular basis.