Driving from Ireland to Russia (St. Petersburg)

Im taking my 16 year old son on a European roadtrip mext week starting with the Nederlands on upt o Denmark, to Sweden, and I hoped from there to get to Helsinki and across by ferry to St Petersburg - either by direct ferry or from Talinn. Can anyone advise me please on the best and most hassle-free route to do this? And another question - Id like to drive on to Moscow and then back to Lithuania and homewards from there - advice?
Thank you very much in advance for taking the time and trouble to think about htis.

Rita