Sadly, good pub sessions are declining in recent years. The trend seems to be towards "Open Mic" nights and similar or booked performers with PA etc.

Possibly, this is down to noisy pubs and the management's desire to attract more punters(Though they usually tend to prefer big football screens)but there's also a tendency for many musicians and singers who just want to be heard themselves as opposed to wanting to share the music with others.

Also, many of those sessions which do exist seem to attract singer/strummer types and/or afficiandos of American music. :-(

There are, of course, still some good places worth seeking out. Moniaive Fesival(Now past, second weekend in May) is a great weekend too.
Also, there's a few good places "Up North" but these are eiher just weekly or even more irregular.