If you have to book a flight out of China to get a visa then don't look for the cheapest ticket you can find (which probably would still cost a good few quid as Int'l flights from China aren't cheap (even just to Hong Kong) ... Instead buy a fairly expensive ticket that you can cancel and get a full refund .. that's likely the best deal even if there's a small service fee associated with a refund .. I have done this in the past for Visa applications and it's worked fine - try Cathay Pacific's website
As regards whether you need one or not - I think the Chinese embassy in Dublin requires evidence of travel arrangements, but I'm not really sure how strict they are ... I know in the past I have exited China via the KCR train to Hong Kong with no tickets bought in advance - but it's a few years ago so I've no clue what I put on the visa form at the time (I guess I must have had a flight form HK - Europe and probably noted that, but not sure)