No tunnel or bridge, but various air and sea crossings -
by sea to N Ireland:
from Campbeltown in Argyll by rib to Ballycastle (summer only)
By ferry from Cairnryan to Larne
Think those are only two routes from Scotland (unless there's still a Troon to Larne summer ferry?)
Not sure of the crossing(s) from England to Ireland - from Liverpool?
Neither tunnel nor bridge seems like a good idea as theres enormous amounts on munitions and military waste dumped in the Beaufort Trench (short sighted decisions by the powers that be...)