If what you are looking at is only the legal implications, then perhaps you will not have a lot to worry about. However as Brendan points out "if you give your word then you should stick to it".
What if the shoe was on the other foot? I guess you'd not like it too much? You are prepared to accept money to do a job and not complete - not nice. In any case this potential Dublin job, may not come off, you may be able to delay taking up any new position, or in worst case at least have the gumption to have a backup lined up to complete the contract you promised to the London outfit.
Unless of course the job in Dublin involves politics when breaking your word might be an honourable thing to do.