You ask how townlands were laid out. I am not really familiar with the US system so can’t really comment on how similar they are. First thing I should say is there is a parish of Easky, and also within that parish, a townland of the same name. So you need to be clear which it is your family have given. Each townland can be found on the Griffiths maps, and are delineated in large red lines. The parishes are not shown on those maps at all.
The townland was the basic unit of land administration. Road names and house numbers only came to rural Ireland in the 1950s. Prior to that your name and your townland was all that was required to deliver a letter to you. The postman and anyone else who needed to know, would know where each person lived.
The origins of townlands are lost in the mists of time. They go back to the time of the Normans if not earlier. They are a system of dividing up land so that in theory there was a roughly equal share of good, bad and mediocre land in each townland (though in practice that was not always so).
The boundaries were defined and often followed natural features like rivers and hills etc. Many townlands had several names. In the 1820s at the tithe applotment assessment the names were standardized and as far as possible any boundary disputes were resolved.
At say, the 1820s, most of the land in Ireland belonged to 10% of the population. Mainly Anglo Irish aristocracy, though there were some big Irish landowners too. They had mostly been given the land by the King in return for loyalty and other obligations (such as raising troops, administering justice etc). These big landowners then sublet, and very often there would be further sublets down the line. (In Griffiths Valuation you see the name of the lead tenant for each property on the left hand side of the page, and his/her landlord on the right hand column. That landlord may not be the owner of the land, just the next tenant up the lease chain. (And most land was leased rather than owned. Where it is owned, Griffiths described it as “in fee”.)
When Griffiths’s clerks did their valuations, they marked each holding on a master map and gave each property a little number (which you have noted). Where say a big farm had several small houses nearby let to weavers and farmhands, these would often appear with a suffix eg 5 for the farm, and 5a, 5b, 5c etc for the smaller households. Property with a rental value of less than £5 a year was excluded. Sometimes Griffith’s clerks included them anyway but left the valuation box blank (or showing nil rental). In other cases they ignored those smaller properties.So there could be other houses on a townland that are not listed at all.
When Griffiths was put on-line they seemed to have used a mixture of maps. Some are copies of the Griffiths clerks maps and so show the location of each property, and others have no properties shown. I don’t know why it was put together like that but presumably at the time they couldn’t get their hands on all the maps showing the locations. Where I live in Ireland (County Antrim) the local libraries have all the Griffiths maps, so if I need to find where a particular property is, I look it up on the library copy. I presume Sligo will library will have the Griffiths map of Easky, if you want to find out exactly where specific properties are.
Griffiths Valuation was done in the years 1847 – 1864, gradually, working across the whole country. The date is shown in the column marked “details”. Easky townland was done in 1857. Roughly every 10 years after that there was a revaluation of each household and you can use those revaluation records to find out how long someone stayed in a particular property, and who replaced them etc. The year of any change is marked in the revaluation book.The revaluation records are not on-line. I assume those for Sligo are held in the National Library in Dublin.
People didn’t travel far within Ireland in the early and mid 1800s. Most had no horse, the bicycle and railway train hadn’t arrived (as far as I can tell the first railway arrive din Sligo in 1863), and it was very expensive and slow to travel by coach. So most people walked everywhere and rarely ranged more than a few miles from where they were born. They therefore married someone locally, and stayed locally and so where you see the same family surname in an adjacent townland, there is a strong likelihood it’ll be the same family, though the connection could obviously be generations back.
If you want to see the parish boundaries, then this site will show you where each was. (I assume your ancestors were RC). Be aware that many RC parishes have different boundaries and different names to the civil parishes.In the case of Easky, the only difference appears to be that the RC parish name has an extra e, ie Easkey.
Can’t comment on your family secrets, but most people left for a one or all of the following reasons: too many people on too little land (there was a huge population explosion in Ireland in the period 1740 – 1840 with the population rising from 3 million to 8 million); famine; repressive land ownership laws and for general economic betterment. There was much better paid work in places like Britain, the US, Canada, Australia etc. Ireland has few natural resources, like iron ore, coal, oil etc and so didn’t experience the industrial revolution that other countries did. So they headed to places like Pittsburgh for mining, steel making, or Glasgow to build ships. And so on. There was no opportunity to obtain work like that in Sligo, where the main occupation was subsistence farming on tiny farms. I expect a few left because they were wanted by the law though.