Well I presume they are done in single print runs because as you will notice Statute, like the printed version of the Constitution, does not come in two versions one for each language. It is a combined version with all the left hand sides of pages being in Irish and all the right hand side being in English. However, obviously you would use half the amount of ink and half the amount of paper if you only printed in one language but I really don't think it would be half the cost given the above. Beyond which, I imagine people like me, who have to go and buy copies of the Acts of the Oireachtas more than cover the cost of the entirity of the print run. Buying the Companies Acts 1963-2006 costs well over 100 euro to purchase in the Government Publications Office whilst it costs a fraction of that to print. Even small legislation like the Succession Act costs around a tenner.
Similarly for road signs. I would imagine the greater cost of a Road Sign is purchasing the sheet of metal and erecting it. Adding an additional line of text is probably a very small cost.