The second picture of the Grainan shows the single entry door and a smaller portal low on the wall. This portal opens into a small passage that runs round the inner wall to a wall forming one of the walls of the doors passage. The portal is about 2 foot high but opens up to about 3-4 foot inside getting gradually larger until it reaches the inside of the doorway passage.
This doorway passage is about 8-10 foot (I think, didn't measure) long as it is the full thickness of the wall. If you go into the small passage (bring a torch, its pitch dark) and follow the curve round you will find a little light at the end where gaps in the rocks in the wall at the end let light in from from the main passage. Some gaps are big enough to stick a hand out. From the main passage looking at the wall you would have difficulty finding this hole.
There is a similar portal and passage curving round on the right hand side of the fort (looking out through the door) but the end wall is solid. This entrance is smaller.
Inside both of the passages its damp and cool with a loose rock floor. The roof is uneven so bring a torch or you'll skull yerself! Don't think its encouraged to crawl round these passages as they would not be considered safe.
Not sure what these passages are for as they must be part of the rebuild from the 1800s. There appears to be one opposite the main door but having seen the old rebuild plans which show both the above passages I think it may just be missing stones or a drain.
The fort itself (while rebuilt) is one of the most impressive in Ireland and commands an unrivaled view over loughs Foyle and Swilly. Just down from it is a spring (called a holy well, but most springs in Ireland are). An amazing place to be on a sunny day!