Old Cemetery in Merrion Square

I've never heard of a graveyard in the square, it was laid out in the mid 1700s, none of the historic maps since that time (Rocque 1756*, Watson 1783, Thom's, P&O, OS c1840, c1900, c1930 etc) show any indication of a graveyard. Access to the square was restricted to resdents only until the 1970s.

*The Rocque 1756 maps shows the square as 'Kildare House Gardens'
 

Thank you Shanew147 for your reply and info.