This is a fairly common phrase, though phrasing constructs are a little more relaxed in the Uk and Ireland than most other English speaking areas.
something is "A tiny bit of an ask" if it is something that would be considered not expected of someone but you ask them to help you anyway or if you are hoping to be lucky or achieve something you may not entirely deserve.
It does not have a literal translatable meaning. Its a mixture of social conventions, whimsy and euphemism.
If i make a bet on 10 coin tosses and only need 5 wins to win then I have a 50%
of waliking away with the winnings. If I however lose the first 4 tosses then I need 5 out of 6 wins, which is a bit of an ask. In other words it is not very likely but it is still possible. I would be asking chance/fate to do me a favour.
Calling it "a bit of..." is simply a turn of phrase. It is a soft euphemism for "too much of..." or "quite a large...".
Some of this comes from the politeness of the culture. If it was genuinely a small ask(favour) it would not be mentioned, maybe instead someone would say (hoping for the best, we will see how it goes) but if its a complete improbability is "a bit of an ask, you should keep your expectations in check"