I wouldn't necessarily assume that people joining in would be welcomed at all the sessions mentioned in the above posts. Generally speaking the size of the welcome will be proportional to the musical ability of those wishing to join. A bit of discretion is advised!

By the way unfortuantely The Lighthouse in Howth is by all accounts being redeveoped, but not as a pub. A number of other Pubs in Howth are also under threat of being converted into more lucrative appartment blocks. As far as I know the Pier House remains a venue for sessions, at least for the moment.