One of the big advantages of a well-thought-out placecode or postcode system in Ireland would be for creating databases of local services.
You would be able to figure out what public transport to take between two addresses just by keying the two numbers into a database. Equally, you could look up an events database of shops or things happening within one or two placecode districts of where you are.
You could in principle do the same thing with a system like geodirectory or using map coordinates. In practice, though, it would be very complicated and expensive.
In contrast, the placecode system could be implemented inexpensively, using simple database tools. A simple lookup table could show the distances between pairs of codes to provide decent geographical information services. It would also make postal addressing and delivery a lot more accurate, especially in rural areas, where a postcode could eliminate the problem of non-unique addresses on long, unnumbered roads.