The best way to get there is to get to Dublin, then use the festival bus to the site http://www.buseireann.ie. It costs around 25 Euros for a return ticket.
You can also get from Belfast by contacting firstname.lastname@example.org. You can also get a train to Portlaoise train station which is a 15 minute drive away, or get a festival bus for 4 Euros to the site, though there are not many running (check www.irishrail.ie for information).
latest on this festival
festival home page
line-ups & rumours
Electric Picnic 2017 Review