Certainly within walking distance - I reckon about 35-40 minutes taking the short cut through Clonard out on to Ballinteer Road, on to Ballinteer Ave and right to Marlay Park.
You could also go through Clonard, take the short cut again down Wesley Lane and get the 48A bus which would take you to the Ballinteer Community School stop in Broadford, which is about 5 mins walk from Marlay Park. If you are lucky with a bus the whole trip should take 20 mins or less.
PS - Point of interest (not being smart) but most people don't realise, Marlay is spelled ending in 'ay' not 'ey'