I work in East Point and asked the management company to share their routing for a 10 min walk from O2 LUAS, they never got back to me! It's a 20 min brisk walk (I've done it in 15 mins when pushed but I'm an extremely fast walker.). Shuttle bus makes most sense if travelling at peak times when the buses are frequent.
The pedestrian route linked to above shows a non-optimal route to the front gate. Also, depending on exactly where in the park your office is, the back gate may be better, as ontour suggests.
The best routing for either gate from the O2 LUAS stop is to immediately head east onto East Wall Road.
For the front gate, veer left at the junction to stay on East Wall Road and eventually turn right onto Alfie Byrne road. There is a small pedestrian shortcut through the park over the mouth of the port tunnel.
For the back gate, cross Promenade Road onto Boundary Road, as ontour suggests. Some of the industrial land bordering the road does communicate with the park before the back gate, but it is not safe (or legal) for pedestrians to use this as a shortcut.
Another option if you find yourself at Connolly or Busaras is to take almost any northbound Dublin Bus to Annesley Bridge (North Strand) and then walk to the front gate. Though if the weather is bad DART from Connolly is better as you can then connect with a shuttle bus at Clontarf Road and minimise walking - but at off peak times Dublin Bus to North Strand is quicker than DART, due to better frequency.
The quickest option to connect with a train from Heuston is actually to cycle, either bringing a fold up on the train, or parking a bike overnight in Heuston.