I listed random mythical subjects related to The Secret of Roan Inish below, to give you a rough idea of the movie itself(that is, if you have not seen it):
-The Irish mythology/"folklores"
a) the Irish tales about Selkies, seals that can shapeshift into humans at will; sometimes live among the humans, or in fact have children with humans, until they miss the sea too much& leave the humans behind (think mermaids)
b) "the sea" as "the mysterious"-- not exactly an expert on Irish mythology-- but probably something along the lines of "mother sea", "the dark sea"(and perhaps both, since the Irish depended on the sea- I would not be surprised if it's both benign& malign, which, according to my view, closely resembles that shamantic hunter-sentiment, esp. noticeable in animal portrayals). Very interesting is that the sea is also portrayed as somewhat human-like, for example, "[the sea grew disgusted/angered and spit out the fisherman (who would have drowned otherwise)]"
There's also an idea that "the sea and land are one"; also, at least for the protagonist's family, the sea is friend.