The DUP leader said he has written to President Michael D Higgins to seek a reason for his decision to decline an invitation to attend a church service alongside the Britain's Queen Elizabeth II to mark Northern Ireland’s centenary.
Sir Jeffrey Donaldson said many people would regard the step as “a snub” in terms of reconciliation and would assume “politics is at play”.
Unionists and the Alliance Party have asked Mr Higgins to reconsider after it emerged he would not be present at the event in Armagh next month.
It is understood the queen has also been invited to the service, which is a focal point of a programme of events in 2021 to mark 100 years since Ireland was partitioned.
A spokesperson for Mr Higgins said he was “not in a position to attend”, but did not explain why.
Mr Donaldson said: “I think it’s very unfortunate that for a service that the focus of which is hope and reconciliation that the head of state of our neighbouring country feels that he is unable to attend.
“We don’t know the reasons why that is the case. However, given that the President of the Republic takes advice from the Irish Government on such matters, one could conclude that perhaps there is some politics in all of this.
“If that is the case, I think it is most unfortunate, particularly given the progress that we have made during the period of the World War One centenary when we all worked hard, including President Higgins, to take an approach which was respectful of our shared history.”
Asked if Mr Higgins' move was a snub, Mr Donaldson said: “Without knowing the reasons for his decision not to attend, it is difficult to say.
“I think it would have been better if we had clarity on the reasons why and in the absence of such an explanation it is difficult not to conclude that there is politics at play here and many will see that as a snub in terms of marking the occasion of Northern Ireland’s centenary.”
He added: “We have written to President Higgins seeking an explanation as to why he is unable to attend the church service. Minister (Simon) Coveney will be here later in the week. Perhaps he will be able to offer some more detail why it is the case that the Irish President is not able to attend the service in Armagh.
“I have encouraged President Higgins to change his mind and to think again about this decision.”
A spokesperson for Mr Higgins said: “The President is not in a position to attend the ceremony you mention, and this has been communicated to the organisers.
“The President, through his office, has already conveyed his good wishes to Her Majesty Queen Elizabeth II.
“The President has welcomed, and continues to welcome any opportunities to meet with Her Majesty and members of her family.”
Alliance Party MP Stephen Farry said he hoped Mr Higgins would reconsider his position.
“This event is very much in keeping with the reconciliation efforts of successive Irish presidents and the queen over recent years, and the inclusive approach to marking the decade of centenaries,” he said.