There is a fantastic biking road around Carlingford Lough offering great views of the Mourne Mountains to the north and the Cooley Mountains to the south. The A2 is a delightful route squeezed between the Mournes on one side and the sea on the other. The Victorian seaside resorts of Rostrevor and Warrenpoint are pleasant stops along the Mourne Coastal Route. Newry is worth exploring with its canal that once carried goods to and from Belfast via Lough Neagh.
On the Omeath side of Newry is the Victoria Locks that connects the ship canal and Carlingford Lough. The R173 wends its way to the characterful Carlingford with its several castles and harbour.
After Carlingford village, the road runs to Greenore and thence to Dundalk.
The A2 road from Coleraine climbs over the mountain to reveal the expanse of Lough Foyle below. The 9 mile long Benone Beach flanks the Atlantic Ocean curving around Magilligan Point into the lough.
A seasonal ferry service operates across the Narrows to the busy deep sea fishing port of Greencastle (check times!). Ride into Moville and then over the mountain towards Carndonagh, turning off in Gleneely to the little village of Culfdaff with its beach and river of the same name. Malin town is a few miles away, a neat little place built around a village green. The road to Malin Head runs in a loop around the Malin peninsula starting near the coastguard station. Malin Head is the most northerly point on the Irish mainland and there are fabulous views across to Inishtrahull island 6 miles distant and the Scottish isles beyond.
The alternative route to Inishowen runs through the walled city of Derry-Londonderry and thence along the shore of Lough Foyle to Moville.
Weather! As with any coastal area, be prepared for some rain! One Loughs Agency biker maintains that the coastline around Malin, Benone and Carlingford Lough regularly enjoys bright and sunny weather!
Law! These routes wind across the border between the Republic of Ireland and Northern Ireland (largely follows UK law and driving regulations but there are some local peculiarities). The entire area is, of course, within the EU.
Telephones. There are a range of mobile 'phone service providers. Watch out for roaming charges, even when you think you are not close to the border!
Currency. Euro in ROI and Sterling in NI - businesses in border areas often take both and automatic bank machines sometimes offer a choice in dispensing cash.