If, when you tune the Dublin ATIS and you here that the Trans Level is FL60... does this mean that you cannot cruise at FL55 and indeed the next available cruising level for the little cessna is FL75 (which may be out of reach of the climb performance if max weight!!)
It's probably worth pointing out here some basic altimetry, as what you've written doesn't actually happen.
Transition
LEVEL is a function of Transition Altitude and QNH. It doesn't mean, as you say in your examples, that you can't fly in the levels that are between Transition Altitude and the Transition Level - what it means is those levels don't actually exist!
Example: QNH is 977mb, Standard is 1013. That's a 36mb difference or, another way to look at it is 1080
feet (30ft / mb).
5000ft + 1080ft is 6080ft or FL60.8.
So - 5000ft QNH
equals FL60.8
First available FL (Transition Level) is therefore FL65 (you always round up).
Conversely - go the other way:
QNH is 1037, difference from 1013 = 24mb or 720ft
5000ft
minus 720ft equals 4280. Round it up, transition level equals FL45, which is 220ft
above 5000ft on the QNH.
So - you can fly at any level you like up to 5000ft - (or whatever your local Transition Altitude is - for those of us in the UK mainly 3000ft but there are local differences for certain Control Zones and TMA's) but above that it's time to do some mental maths.
Rule 1 - don't fly at Transition
level. Depending on Pressure it can be close to the Transition Altitude. First available in the UK is normally 500ft above TL. That guarantees you at least 500ft separation as provided by the quadrantal rule in the UK.