Starting his career in the Sligo Rovers youth system, the 23-year-old made his first team debut for the Bit o’ Red under Paul Cook in 2011 as a late substitute against Dundalk.
He broke through into regular first-team contention in 2013, making eleven appearances in the League of Ireland Premier League before moving to Ballinamallard in Northern Ireland.
Martin made eighty-seven league appearances for Ballinamallard in the Premiership Northern Ireland, scoring 13 goals, before moving back to Sligo in 2016.
After making a further forty-eight appearances for Sligo, his contract was cancelled by mutual consent after seeking a move to England.
Martin has made several appearances for Port during pre-season, impressing the management team enough to earn a permanent contract.
Manager Alan Lewer told the club`s official website: “Liam’s a really flexible player, he can play out wide or off the front man. He did very well over in Ireland. He played under Paul Cook in Ireland so we had a chat about him when we played Wigan. He’ll run through brick walls for the team, he’s got great character, and we know he can score goals”.