If your flights are on the same ticket the your bags will be checked through and all you need to do when you arrive in Heathrow is follow the signs for Flight Connections (they're in purple so you can't miss them) and take the transfer bus to Terminal 3.
If you've booked your Aer Lingus flight separately you need to claim your bag and then exit the security area and follow the signs for Terminal 3. Don't worry, it's quite easy. When you come out of Terminal 2 you will cross over a wide walkway bridge and there will be a bank of lifts to your right. Go down to the level marked Underground and I can't remember what else and from there follow the signs. You'll pass the entrance to the Underground and the signs to T3 will be very clear. It's a subway passage and at the end you'll come out in front of T3. The entrance to the Virgin Atlantic check-in area is to your left.
Enjoy your trip. You'll be just fine
