McGregor is considering a legal challenge to the eligibility rules for presidential elections, which mandate nomination by 20 members of parliament or four local authorities.
Mr O'Brien acknowledged McGregor's right to challenge the process but reiterated that the Irish constitution's nomination process is clear and he does not expect McGregor to be nominated.
The minister contrasted the distinguished service of current President Michael D Higgins with what McGregor might offer, emphasising the critical importance of the Uachtaran na hEireann office.