The onsale date for tickets, has not been announced yet, that information should be available tomorrow, when the gig is officially announced.
Going on the usual arrangements, i would speculate tickets will be onsale sometime next week, but lets wait to hear the official announcement first.
Remember, if you know someone in Ireland who has an O2 mobile account OR a 3 Mobile account, they can get you the code required to participate in the pre-sale, which is usually
2 to 3 days before the public onsale.
It appears the presale ticket access will initially be shared between "O2" and "Three" mobile customers, meaning pre sale tickets could be harder to nab, than was the casebefore, when it was only available to 02 mobile customers.
3 Arena website is
http://3arena.ie/