The speed limit bye-laws https://www.wicklow.ie/Living/Services/Roads-Transport/Road-Policy/Speed-Limits refers to it as Drumkay Road (L1101), as o a few other sources.

This is important: https://www.sabre-roads.org.uk/wiki/index.php?title=Talk:R999 There is no such thing as the R999 - I think this may have been a template sign that was on the construction drawings, but no order has ever been made assigning R999 to anything. 

http://www.irishstatutebook.ie/eli/2012/si/54/made/en/print
http://www.irishstatutebook.ie/eli/2018/si/435/made/en/print
http://www.irishstatutebook.ie/eli/2019/si/577/made/en/print

I suggest that the name not be applied any further east / south than the first private entrance at Murrough (street name) and that any road named Murrough should not have its name changed..