The substitute for Fianna Fáil MEP Brian Crowley, Kieran Hartley, has expressed disappointment at his party leader, whom he says has refused to address his calls for Mr Crowley to step down if he is unable to return to the European Parliament.
Brian Crowley was elected as MEP for the Ireland South constituency in 2014, but has been absent from parliament for the last three years due to illness.
A spokesperson for Mr. Crowley said that he has been in Cork University Hospital, recovering from a series of hip and leg and operations.
Speaking to East Coast FM's morning show, Mr Hartley said he has written to Micheál Martin on three separate occasions and has been met with a wall of silence.
"I believe at such a crucial time that Brexit is happening, that right now the Irish people need representation and need people in the European Parliament representing their views the opinions, and having a say on what happens in what will probably be the most difficult time for Ireland, when our biggest trading partner is about to leave the European Union," he said.
"I do believe that this has actually questioned Micheál Martin's leadership skills."