I've just had a trawl through the Irish market for rib insurance - magic speed seemed to be 40. Obviously no-one knows exactly how fast your kit is, so any realistic figure will be accepted - find out where the penalties kick in!
I may be wrong about this, but my understanding about the trailer is that the rib policy covers the actual rib and trailer under tow, but your car policy covers any third party damage or injury - so check the car policy - I've seen what a "loose cannon" SIB can do. Luckily, sheep are cheap......
Allianz Direct where cheap, if a bit ditzy - and their liability cover was much lower (by over a million euro) than the others. General advice seems to be 2.5 mil cover, us being so fond of the High Court and that.