If you're confident of selling - and any property within walking distance of Dublin city centre is likely to be that - why not wait until closer to the time? Unless you think the market is likely to fall, you will not only be more likely to achieve a higher price, but you will minimise or avoid completely any interim rental and/or storage costs (and rental is likely to be both expensive and troublesome in the current market).
If I were you, I'd put it on the market in September, after the summer lull, to give time to complete the sale by the end of the year. Apart from anything else, you will have more visibility on whether or not the new house is likely to be ready in time.