On December 7th, Mars and Neptune will share the same right ascension, with Mars passing 0°02′ to the north of Neptune.
At around the same time, the two objects will also make a close approach, technically called an appulse.
From Cork, the pair will be visible in the evening sky, becoming accessible at around 17:04 (GMT) as the dusk sky fades, 27° above your south-eastern horizon. They will then reach its highest point in the sky at 18:28, 30° above your southern horizon. They will continue to be observable until around 22:40, when they sink to 10° above your south-western horizon.
Mars will be at mag 0.1, and Neptune at mag 7.9, both in the constellation Aquarius.
The pair will be close enough to fit within the field of view of a telescope, but will also be visible through a pair of binoculars.