just my two cents' worth:

The High Kings, as a music-making entity, was a business proposition from its inception, and this is transparent in promotional publicity, press releases, interviews, and so on.
If the four co-founders knew each other personally before The High Kings were made official, I am not aware of this, and it would interest me to hear differently from anyone who knows better.
All four original members came from musical families and had professional careers as musicians before the promoters and businessmen brought them together.

I wonder how many of the fans appreciate the grueling nature of the international tour itineraries to which the High Kings were committed for those initial ten years. Personally it fills me with dismay to look at those schedules, I doubt that I could possibly have stuck it out for ten years as Martin Furey did.

Was the initial agreement and contract for ten years?
Probably not, don't you think --
more likely the period of the initial legal agreement was shorter,
and then all four, at the expiration of the agreement,
committed to join in a new contract for another period of time.
However many agreements there were during the ten-year history,
Martin Furey saw the agreement of that moment coming to its expiration date, and decided not to renew.
No press-worthy drama here, the artist had simply had enough.