The 27-year-old midfielder, who has six caps for Ireland, has agreed terms with St Pat's for the 2016 SSE Airtricity League season.
Keith spent his youth at Belvedere and began his professional career at Blackburn Rovers.
He had loan stints at Stockport County and Sheffield United and spent a year at Preston North End before joining Burnley in 2011.
He moved to Barnsley in 2014 and returned to Ireland in 2015 and played 6 games for Drogheda United.
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