The Blues developed an early 10-point advantage following a Nick McCarthy try and five points from the boot of Steve Crosbie, before Rotherham opened their account with a ninth-minute Jordan Davies touch down.
The hosts responded with a breakaway try by winger Mick McGrath, but a 27th-minute Max Argyle five-pointer meant Leinster had to be content with 17-10 interval lead.
But Leinster ‘A’ ultimately eased home with tries from Crosbie, Cian Kelleher, Darragh Fanning and McGrath.
M McGrath 2 tries, N McCarthy, C Kelleher, D Fanning try each, S Crosbie try, pen, 2 cons, J Carbery 3 cons.
J Davies, M Argyle, J Redfern, D Harris try each, W Goodwin 2 cons.
B Dardis; C Kelleher, R O’Loughlin, H Brewer, M McGrath; S Crosbie, N McCarthy; J Loughman, B Byrne, O Heffernan; D O’Connor, J Ryan; J Murphy, D Leavy, P Timmins.
A Dundon, A Porter, W Connors, T Ryan, C Rock, J Carbery, D Fanning.
S Scanlon; M Keating, J Davies, J Barker, J Hayes; R Jones, L Carter; R Hislop, J Preece, M Tampin; T Holmes, B Maddison; M Argyle, T Calladine, J Redfern.
D Harris, T Williams, J Elliot, G Oram, A Birch, J Gill, W Goodwin.
D Jones (WRU).
Bristol 47 Ulster A 13.
Nottingham 19 Cardiff Blues 10.
Castres 24 Pau 7.
Cardiff Blues 74 Calvisano 6 European Rugby Challenge Cup,
La Rochelle 35 Worcester 11.