I'm not sure if it was clearly explained that it was certain monasteries on Mt Athos that have this practice. When I was there it was only a few monasteries which did this and they were usually those associated with Philotheou monastery. In other words this is not standard practice in Orthodox monasteries. Although we must all be respectful of and understanding of the understanding of place which is still so widely practiced in Orthodoxy.
Thus even with what for us may be more difficult practices, such as having non-Orthodox visitors eat apart from the main trapeza, we still need an understanding of why they do this. I personally had trouble with this practice when I saw it, it troubled me & I would see it as being harmful in our local context- but then again, what if at this monastery they see every aspect of their lives as being sacramental? And thus, like any Orthodox parish, would only allow Orthodox (and often it's only properly prepared Orthodox) to approach the Cup. So the idea then is not to be demeaning of a person but rather to relate to them in that way which reflects God's placing of them and amidst His order.
In Christ- Fr Raphael
Kelil since you're in Ireland why don't you go and visit an Orthodox church there? I'm sure you'd find them to be very welcoming. Personally, I think anyone who's not Orthodox should begin their journey by attending a local Orthodox church where possible so that they can get used to the customs and traditions that we Orthodox hold before jumping into such a highly ascetic experience like a visit to Mt. Athos, although it is possible for a non Orthodox person to be totally transformed by a pilgrimage to such a place.
Here an Anglican priest embarks on a pilgrimage to St. Anthony's in Egypt and undergoes an amazing transformation for the short time that he's there: http://blip.tv/file/1762501