"They are unlikely to go underground, sideways would be the best bet."
Don't know where you got this idea , I regularly deal with the ESB , both networks and engineering as this is a problem on a lot of our firms sites.
In all cases the esb will put them underground.
The cost to you will depend on the following, voltage of existing lines, available space on your site for ground level transformer, future network development plans.
You need to speak to the esb asap. They're not that bad to deal with imho, DON'T LET THEM DO THE WORK AND CHARGE YOU FOR IT.
Its a simple ducting operation, arrange a wayleave with the esb away from the footprint of your building, agree the line the ducting will be running along , agree the size( typically 125mm pink esb duct) and number of ducts required and the size of the drawpits. All this information will be given quite freely by the esb who will in turn send their own clerk of works to inspect your ducts etc prior to them running cables through them.
Be aware of their own regs in relation to working adjacent to overhead cables if you do go down this route.
If you pm me with the size of your site i can throw a few ballpark firures back at you for labour , plant and materials to do the work, again i can't stress enough DON'T LET THE ESB DO THE WORK AND CHARGE YOU FOR IT, they'll only get a contractor to do it and add x% onto his/her invoice.