I'm just gonna go for London and then when Dublin is announced just gonna hope for the best in the general sale. If London is getting 4 gigs, so should Dublin. 3Arena I reckon.
From U2.com tours FAQ:
Q - If I don't use my presale ticketing access code in presales for shows announced on December 3rd, will the code still be valid for use if/when other 2015 dates are announced - such as the Dublin shows, details of which are still to be revealed ?A - Yes. Your un-used presale ticketing access code will be valid to use in further shows announced for 2015.[/size]