So you can see Ireland and the Llyn.. but what else can you see from up here? Chapter 2 of John Rhys's 'Celtic Folklore, Welsh and Manx' suggests the following:
Mr. E. Perkins, of Penysgwarne, near Fishguard, wrote on Nov. 2, 1896, as follows, of a changing view to be had from the top of the Garn, which means the Garn Fawr, one of the most interesting prehistoric sites in the county, and one I have had the pleasure of visiting more than once in the company of Henry Owen and Edward Laws, the historians of Pembrokeshire:--
'May not the fairy islands referred to by Professor Rhys have originated from mirages? During the glorious weather we enjoyed last summer, I went up one particularly fine evening to the top of the Garn behind Penysgwarne to view the sunset. It would have been worth a thousand miles' travel to go to see such a scene as I saw that evening. It was about half an hour before sunset--the bay was calm and smooth as the finest mirror. The rays of the sun made a golden path across the sea, and a picture indescribable. As the sun neared the horizon the rays broadened until the sheen resembled a gigantic golden plate prepared to hold the brighter sun.
No sooner had the sun set than I saw a striking mirage. To the right I saw a stretch of country similar to a landscape in this country. A farmhouse and outbuildings were seen, I will not say quite as distinct as I can see the upper part of St. David's parish from this Garn, but much more detailed. We could see fences, roads, and gateways leading to the farmyard, but in the haze it looked more like a panoramic view than a veritable landscape. Similar mirages may possibly have caused our old to think these were the abode of the fairies.'
Yes you can see both Ireland and the Llyn as well as Snowdonia and Cadair Idris, on some days all are visible. Garn Fawr dominates the area and surounding it are ancient possibly Iron age field systems. At it's base on the Pwll deri side opposite the Youth hostel in the farm yard there is an ancient corbelled hut some have suggested medieval but it seems much older reminiscent of similar structures on the West coast of Ireland, although probably utilised in the Medeival period.
I've not yet visited this site, but apparantly it's one of the best preserved stone hillforts forts in Britain. Access to the fort is good (there's a small car park) and on a clear day you can (allegedly) see Ireland and the Llyn.
My sources are Chris Barber's More Mysterious Wales, the NMRW and the Ordnance Survey Landranger.