People in Co Donegal were woken up and their homes were shaken as the north of the county was hit by a minor earthquake.
The rumblings were reported by people in towns and villages on the Fanad peninsula near Lough Swilly in Co Donegal on the north coast of Ireland, the British Geological Survey (BGS) said.
The quake, just after 1am, measured 2.2 on the Richter scale and was recorded at a depth of 3km on land north west of the Buncrana region.
Julian Bukits, assistant seismologist with the BGS, said window frames shook and some people were woken up.
"It happens naturally in the north of Ireland but not as often, say, as northern Scotland or the south of Wales," he said.
"No one can really explain it for Ireland, it's just a geological phenomenon. Usually, with reports of this kind, people think it's a heavy lorry going past their home."
The BGS has narrowed the epicentre of the quake down to a few hundred yards around the Knockalla heights coast road, about five miles across Lough Swilly from Buncrana.
People as far away as Letterkenny, Kilmacrennan and Ballybofey reported the rumblings, as well as more locally in Buncrana, Kerrykeel and Urris.
The BGS had eight reports from people feeling mild vibrations, a loud rumbling, creaking and shaking of windows and doorframes in homes.
No damage was reported.
The last similar-sized earthquake in Ireland was a 2.6 in north Clare in May 2010.
Mr Bukits said: "They are not very often and as far as earthquakes go, 2.2 is relatively insignificant."
There have been eight minor earthquakes recorded in this region of Donegal.