A friend of mine used to commute from Portlaoise to Ballsbridge by train and she used to get the 10 to 7 train to get into Dublin for just after 8 and she would then get a bus and would be in work for around 8.40. Bear in mind that while train might say it gets in at such a time you need to consider delays etc and then traffic when you get off train. You might be better off getting earlier train (i think the first one is at 6am, this gets in around half 7) if you decide to use that mode of transport.