Cosplaying Irish ‘Mythology’
The site of Cú Chulainn’s supposed settlement at Dún Dealgan (which actually means ‘the Dún of Dalga’) was, in far more recent times, anglicized to Dundalk. When the town opened its first museum in 1901, therefore, it’s no surprise that they harkened back to it’s imagined ‘mythology’ rather than its real history – which involved far less appealing aspects like colonisation, land grabbing, subjugation, etc. – to celebrate its opening.
That opening was marked with a huge outdoor pageant with numerous people dressed up in ‘authentic’ fantasy costumes of the period. To celebrate the occasion four postcards were also produced (two of them attached).

Outdoor pageants of this type were very popular at the time but, obviously, come 1916, people suddenly had a lot more on their mind and the cosplaying abruptly fell out of fashion.
Strangely enough, Irish ‘mythology’ cosplay seems to have returned with a vengeance over the last few years. Nowadays, its not that uncommon to see people dressing up as imaginary figures from an imagined ‘mythological’ past. There’s no real problem with that when its done for fun, of course. Provided everyone understand its portrayal is probably just as accurate as that from 1901.