The answer to this puzzle is:
LOUGH (the Irish word for a body of water, like a ‘lake’ or ‘loch’)
This is supported by Hint 4, since the 5-letter word 'lough' is a quarter as long again as its 4-letter counterparts from England ('lake'), Wales ('llyn') or Scotland ('loch').
A ‘mere’ is a type of lake. ‘Mere me’ could be taken to mean ‘I am a lake’...
Cast second I,
This clues the word SLOUGH, which means 'cast off' and can be made by adding the abbreviation 's' (meaning 'second', a unit of time) to the front of the word 'lough'.
(i.e. Cast = second + I('me') = s + lough).
This word was further clued by Hint 2, being one of the many towns of the English county of Berkshire whose names can also be nouns. Others include Reading (e.g. a meter reading), Maidenhead and Ascot, possibly among others...
Beg or bay,
Both of these are names of loughs in Ireland: Lough Beg in Northern Ireland, and the very small Bay Lough in Tipperary, ROI. Note further that as well as being able to describe an inland body of water, a 'lough' can also be coastal and much more like a geographical 'bay' (consider Belfast Lough, for example), stretching a long way along a coastline.
Till softly I,
This clues the word PLOUGH, which means 'till' and can be made by adding the musical abbreviation 'p' (meaning 'softly') to the front of the word 'lough'.
(i.e. Till = softly + I('me') = p + lough).
This word was further clued by Hint 3, since the constellation Ursa Major is known to the English as 'The Plough' (a piece of farming equipment), whereas to Americans it is more commonly 'The Big Dipper' (a rollercoaster) - work vs play, indeed!
Leave with a coat on me.
This clues the word FURLOUGH, which means 'leave' (as in 'a leave of absence from a job') and can be made by adding a coat (or 'fur') to the front of the word 'lough'.
(i.e. Leave = coat + 'me' = fur + lough).
As for the title:
This (as implied by Hint 1) refers to Lough Erne in County Fermanagh, Northern Ireland. This lough is actually two connected lakes, and (oddly) the more northern of the two is known as 'the Lower Lough' while the more southern of the two is known as 'the Upper Lough'! Sounds like someone had their compass upside down when coming up with the names!