Directories/Almanacs can provide information about a vast array of different aspects of society, and as such they can be a most useful source of genealogical information (standard inclusions are street listings with occupier's names, tradesmen, army officers, solicitors, bankers, magistrates, etc).
Most Directories/Almanacs are 19th or 20th century in date, with a smaller number from the late 18th century, the period in which they first appeared. Directories were published for Dublin, some provincial towns and a few covered the entire country.
Most Directories were published annually, therefore it is possible to track family names and locations and make assessments as to how people were managing economically. A word of caution however, all Directories/Almanacs were already out of date by the time they were published as a time lag of a least 6 months (and sometimes longer) existed between record collecting and publication date; all research should factor in this time-lag.