Our visit to Magilligan Martello Tower was whilst working in nearby Limavady and also gave us a chance to enjoy the fantastic nature reserve. One look at the map and you can see the strategic position of the tower, guarding access to Londonderry:
Visiting Magilligan Martello Tower
The only facilities here are a car park and some information boards but the site is well sign posted. The nature reserve that the tower sits in has plenty of well maintained paths and is the largest dune system in Northern Ireland.
My Experience
Some info form the Discover Northern Ireland site:
The Tower is a well known landmark, marks the entrance to Lough Foyle and is one of the most northerly of the towers built all around the coasts of Ireland.
It was one of 74 constructed in Ireland, 40 or so survive. They were placed at strategic points all around the coast and designed to fire on any invading fleet or withstand lengthy sieges. This tower marks the end of a long tradition in Ireland of defensive buildings stretching back over 3,000 years to Bronze Age forts.
The walls are over 9ft thick and built of imported stone. There are three floors. The top floor housed a twenty-four pound cannon gun able to swivel and shoot in any direction. A small furnace was used to heat the shot in order to set wooden ships on fire. The middle floor was the living quarters for one officer and twelve men. Below is the cellar. This is reached by a spiral staircase. There is a water well and storage rooms for gunpowder and food. The entrance to the tower has been changed. An iron staircase now replaces the original wooden ladder.
We had a lovely hour or so wandering around the reserve, enjoying the scenery and summer warmth.
Magilligan Martello Tower Photo Gallery
History
The Magilligan Martello tower was built towards the end of the wars in 1812 and is one of the most northerly of the towers built all around the coasts of Ireland. Originally a 24 pounder cannon was mounted on the top. It was fastened to a central pivot and moved on a circular rail so that it could point in all directions. The tower was built on top of a spring to ensure fresh water in case of a siege. Below the gun platform were the living quarters and the ground floor was used to store powder and ammunition. There is a similar tower in Greencastle, on the other side of Lough Foyle. 
Martello Tower at Greencastle across Lough Foyle
The dressed sandstone is from the Ballyharrigan quarries in Bovevagh and the building is a monument to the craftsmanship of Irish masons at the time. The wall of the tower is reputed to be eleven feet thick and the structure is well preserved, except for the entrance which is blocked for unspecified safety reasons. It stands in a beautiful location looking out to the Atlantic beyond the golden sands of Benone Strand and with a splendid view of Greencastle and the Donegal Hills on the other side of Lough Foyle.
You might think that all this would make the spot a tourist paradise, but unfortunately this beautiful place is surrounded by an army shooting range and Magilligan jail, which make the drive to the tower a rather forbidding experience. At the moment a pier is being built at the point. It will link Greencastle and Magilligan within the year via a car ferry. This is very handy for the local tourist trade and it will save people from Limavady the 35 mile round-trip, as well as opening a completely new route to escaped non-swimming prisoners from the nearby jail.
References
The discover Northern Ireland site had the information listed above as well as general visitor information here.
There is an excellent article on the tower here. The history above is copied from there.