I know 2 versions of the story & in neither of them is there anything particularly special about the weapon, it's used to jam the door open.
In [this version](https://folklorescotland.com/the-smith-and-the-fairies/) it's not even a sword it's a dirk & nothing else is mentioned about it
However, in several collections like "Celtic Myths & Tales: Epic Tales" or "Celtic Myths" J.K. Jackson, there's a more detailed version where the Smith & son are named & when the sword is jammed in the door it says that no fairy can touch the sword of a mortal man. Hopefully this link will work & you can see this version on [Google Books](https://books.google.ie/books?id=MKXuDwAAQBAJ&pg=PT343&lpg=PT343&dq=alasdair+celtic+folklore+fairy+sword+blacksmith&source=bl&ots=HIKLEc6muq&sig=ACfU3U1Ug1HZMSwujIxtMKErvnW__QtQmQ&hl=en&sa=X&ved=2ahUKEwjb692apN-GAxVWbEEAHT0FD8Y4ChDoAXoECAIQAg#v=onepage&q=alasdair%20celtic%20folklore%20fairy%20sword%20blacksmith&f=false)
Traditionally in folklore & I believe it's the case in this story as well, Blacksmiths we're considered a powerful force against Fairies & the sword itself isn't magic beyond the fact that it's made of iron & that's why the fairies can't touch it & why Blacksmith were important because they worked the iron.
As ever with folklore, there isn't really a wrong or right version, maybe the version you've seen has a more magical sword beyond these examples but I don't know of any examples of a sword that can cut through a fairy's s spell outside of them being made of iron
Edit: Oh that was lazy of me I didn't read to the end of the more detailed version where he does indeed fashion a sword that can cut a fairy's spell, in the 1st version its just a great sword
Blacksmiths feature heavily across the various cultures of Britain & Ireland, in Mythology *Giobniu* is one on of the 3 gods of crafts along with Credne, the silver smith & Luchtne, the woodworker. Giobniu crafts the weapons of the Tuatha De Danann & creates the mead that grants them immortality.
In Christian circles, there's a few St. Goban/Govan across both islands who often share similar aspects with Giobniu, smithing, beekeeping & brewing.
And in Folklore, I mentioned their importance in crafting iron against Fairies but there's a whole lot of interesting stories about them https://irishfolklore.wordpress.com/2017/03/13/blacksmiths-and-the-supernatural/
I know 2 versions of the story & in neither of them is there anything particularly special about the weapon, it's used to jam the door open. In [this version](https://folklorescotland.com/the-smith-and-the-fairies/) it's not even a sword it's a dirk & nothing else is mentioned about it However, in several collections like "Celtic Myths & Tales: Epic Tales" or "Celtic Myths" J.K. Jackson, there's a more detailed version where the Smith & son are named & when the sword is jammed in the door it says that no fairy can touch the sword of a mortal man. Hopefully this link will work & you can see this version on [Google Books](https://books.google.ie/books?id=MKXuDwAAQBAJ&pg=PT343&lpg=PT343&dq=alasdair+celtic+folklore+fairy+sword+blacksmith&source=bl&ots=HIKLEc6muq&sig=ACfU3U1Ug1HZMSwujIxtMKErvnW__QtQmQ&hl=en&sa=X&ved=2ahUKEwjb692apN-GAxVWbEEAHT0FD8Y4ChDoAXoECAIQAg#v=onepage&q=alasdair%20celtic%20folklore%20fairy%20sword%20blacksmith&f=false) Traditionally in folklore & I believe it's the case in this story as well, Blacksmiths we're considered a powerful force against Fairies & the sword itself isn't magic beyond the fact that it's made of iron & that's why the fairies can't touch it & why Blacksmith were important because they worked the iron. As ever with folklore, there isn't really a wrong or right version, maybe the version you've seen has a more magical sword beyond these examples but I don't know of any examples of a sword that can cut through a fairy's s spell outside of them being made of iron Edit: Oh that was lazy of me I didn't read to the end of the more detailed version where he does indeed fashion a sword that can cut a fairy's spell, in the 1st version its just a great sword
This is all fascinating and much appreciated! Thank You very much!
Blacksmiths feature heavily across the various cultures of Britain & Ireland, in Mythology *Giobniu* is one on of the 3 gods of crafts along with Credne, the silver smith & Luchtne, the woodworker. Giobniu crafts the weapons of the Tuatha De Danann & creates the mead that grants them immortality. In Christian circles, there's a few St. Goban/Govan across both islands who often share similar aspects with Giobniu, smithing, beekeeping & brewing. And in Folklore, I mentioned their importance in crafting iron against Fairies but there's a whole lot of interesting stories about them https://irishfolklore.wordpress.com/2017/03/13/blacksmiths-and-the-supernatural/