Dwayne Johnson has sent a Christmas message to a Mayo teenager who is battling stage four cancer, encouraging him to rest over Christmas and signing himself "Dwanta Claus".

The Hollywood star took to Instagram on Friday morning to dedicate a video to Tommy Hanley, reminding him to eat and rest over the Christmas period. Johnson describes Tommy's fight against cancer as "inspiring".

"This one's for my buddy Tommy Hanley. Just found out he's a huge fan, 15yrs old, living in Ireland and battling stage 4 cancer," Johnson said. 

"I've never met you, but I already know so much about you and can tell you're an awesome, honorable and strong young man.

"Make sure you eat up dude and get your rest. Those two things are gonna help you stay strong during this fight.

"Stay being the coolest kid and always keep in mind how much you inspire all your family and friends 0 including me.

"We're all pulling for you Tommy! Stay strong and Merry Christmas to you and your family ~ Uncle Rock aka Dwanta Claus."

Johnson, who is Hollywood's highest earner, regularly posts updates from movie sets, as well as from he calls his "iron paradise2, the gym where he spends much of his time training.