Former Ireland Under-20 international Beirne joined the Scarlets after being released by Leinster for the start of the last campaign.
He has gone on to play a starring role for Wayne Pivac’s Llanelli-based region, helping them to a stunning PRO12 title success with a series of powerful performances at lock.
Beirne hopes to one day fulfil his dream of playing senior international for Ireland and has signed for Munster with that in mind.
Scarlets head coach Pivac said: “We are disappointed to lose such a quality player and quality person from the group, but fully understand Tadhg has aspirations to play for Ireland.
“He’s developed well in the last 14 months as a player and we’ve seen the benefits of all the hard work he’s put in since joining the Scarlets.
“Tadhg’s fully focussed and committed to continuing his rich vein of form for the remainder of the season.”
Beirne has signed a two-year deal with Munster and the Irish Rugby Football Union.
His on-field physicality has helped him win numerous man of the match awards with the Scarlets and he currently tops the turnovers won chart in the Guinness PRO14.
Beirne has also scored three tries for the Scarlets, including one against his soon-to-be new side Munster in the PRO12 final last term.
