The university hospital, some 15 miles (24 kilometres) from the explosion, was placed on an emergency footing to deal with "multiple injuries", it said in a statement.
Ireland's premier Micheal Martin called it a "very dark day" as he spoke to the media briefly before heading to the scene.
"The scale and enormity of it, it's such a small community, it means that almost everybody will know on a friendly basis people who've lost their lives," he said.
"It's a very dark day for the people of Donegal and for Ireland."
Martin earlier thanked members of the emergency services who were working non-stop "in extremely traumatic circumstances".
Agriculture Minister Charlie McConalogue, who represents northeast Donegal in the Irish parliament, compared the scenes to events during the decades-long sectarian conflict on the island of Ireland over British rule in Northern Ireland.
"People are shocked and numbed," McConalogue told Irish broadcaster RTE.
"The scenes from the event are reminiscent of the images from The Troubles years ago, in terms of the scene on the ground and the damage and the debris."
Creeslough is around 30 miles (50 kilometres) from the border with Northern Ireland and has a population of about 400 people.
The Applegreen service station is on the N56 road, which loops around the northern tip of the Irish republic.
Applegreen tweeted that the news was "devastating".
"Our thoughts and prayers are with the families and friends of the deceased, those who have been injured, and the wider Creeslough community," said the company.
Ireland's President Michael D. Higgins expressed his "shock" in an official statement.
"This tragedy is a terrible blow to a community that is closely knit and where every loss and injury will be felt by every member of the community and far beyond," he said.
