You could walk out the side of Heuston and get a bus from the stop opposite Dr Steevens Hospital. 25X, 51d, 51X, 66X and 67X will all bring you to Waterloo road.All very frequent at that time, ie no longer than a 5 minute wait until one shows up. It's about a 10minute walk from Waterloo rd to Lansdowne Rd.
If you're train gets in at 08:25, you should be at St Stephen's green by 08:40 maybe 08:50 at the latest, so 09.30 in Lansdowne shouldn't be a problem