The Northern Irishman’s goals have proved key to the Staggies since he moved to the Highlands in the summer of 2014.
The 26-year-old former Cliftonville and Werder Bremen poacher struck 55 goals in 115 appearances during three seasons in Dingwall.
His 23-goal Ladbrokes Premiership haul from last term even saw him finish above Celtic hot shots Scott Sinclair and Moussa Dembele to top the scoring charts.
But that incredible strike rate only alerted the attention of Nigel Clough’s Sky Bet Championship outfit and he has now made his switch south.
County manager Jim McIntyre said: “It was always going to be difficult to hold on to Liam after the fantastic season he had, finishing top scorer in our league.
“We thank Liam for his services to the club and wish him the very best on what is a great opportunity for him personally. I am sure he will be a great success.”