The twin sister of the youngest ever missing person in Ireland claims political interference has prevented her sister's killer from being brought to justice.

Ann Doherty, the twin sister of missing schoolgirl Mary Boyle, is meeting with Taoiseach Enda Kenny in Dublin today.

6-year-old Mary vanished on her grandparent's farm in Cashelard near Ballyshannon in Co Donegal on March 18th, 1977.

In a statement, Ms Doherty says she believes Mary was murdered by somebody known to her because she was going to reveal a terrible secret.

She also alleges there was political interference in the investigation which has prevented the killer from being brought to justice.

Ms Doherty says she believes gardaí have protected Mary's killer for almost 40 years - and that during that time he was, and remains, a danger to other children.

An age progressed photo of Mary Boyle | Image: An Garda Síochana

Ms Doherty will be accompanied to the meeting by singer Margaret (Margo) O'Donnell, a cousin and long-time family friend.

Both Ann and Margo made formal statements to gardaí almost two months ago, saying they were told the identity of the killer on several occasions by someone who knew Mary.

The women also say they have expressed their concerns to gardaí many times through the years but no action was taken.

As part of the campaign to find her sister, Ms Doherty has travelled to Stormont, the European Parliament in Brussels, and Westminster Palace in recent months.

In the coming weeks, the case will be raised in the House of Commons and the House of Lords in London.