Lawyers for Senator Ivor Callely say the Seanad Committee invesigating his expenses claims "portrayed him as a pariah who ripped off the State".
They are challenging the Committee's finding that the Senator misrepresented where he was residing, arguing that Mr Callely complied with rules as set out by the Seanad.
The senator's legal team are arguing his house in Kilcrohane, west Cork, was his "normal place of residence" at the time of his expenses claims.
They are arguing that, under the Seanad's rules, "normal" is not necessariuly defined as one's permanent or principle abode - rather it is a premises used on more than an "ad hoc basis".
Mr Callely's claim is that the Seanad Committee applied the wrong test and that one member of the committee refused outright to apply the correct definition, seeing it instead as an elaborate scheme by the Senator to avoid Capital Gains Tax on his house in Cork to gain €250,000.
The High Court has heard Mr Callely even informed the Seanad during 2008 that, as a result of personal circumstances, he was spending significantly longer periods in Dublin and that he wanted his normal place of residence to reflect this change.
The court has heard he was told that he was only allowed to make a single declaration in a year.