According to the Collins dictionary, cosmology is the the branch of philosophy dealing with the origin and general structure of the universe, with its parts, elements, and laws, and especially with such of its characteristics as space, time, causality, and freedom. (1)
In Irish cosmology we see quite a few things on Land, Sea and Sky.
A few more knowledgeable people than I have written about these concepts so I am going to defer to them here, after all why reinvent the wheel?
For sources on the three realms please read Liam Mac Mathúna's Irish Perceptions of the Cosmos.
For more about Land (Talam), Sea (Muir) and Sky (Nem) please read the following pages from Tairis:
Many cultures have creation myths, in the case of the Celts and by extension the Irish things are not so clear cut. Again I am going to defer to Tairis for a well written explanation:
(1)"cosmology." Collins English Dictionary - Complete & Unabridged 10th Edition. HarperCollins Publishers. 7 January 2013. <Dictionary.com http://dictionary.reference.com/browse/cosmology>.