A person who claims to be a passionate follower of a sports team, even though their active support is [confined] solely to the TV screen of a public house or bar.
A variation on the theme of armchair supporter.
Commonly found throughout Ireland, usually associated with certain English and Scottish teams, and almost always in possession of a Sky viewing card.

[Barstooling] : Verb (bar stool-een)

Partaking in [the activity] of following your favourite sports team solely from the confines of a pub.