I love Irish linen sweaters. Like alpaca, they make a garment that's warm when it's cool and cool when it's warm. Ralph Lauren's Purple Label has linen sweaters once in a while but generally they're not easy to find, and that's probably because the best source in the world is at the end of it.
Inis Meáin is the middle of three Aran Islands lying 15 miles off the west coast of Ireland, which makes it about as far West as you can go on land from Europe before you get to North America. It is just three miles across, with a unique landscape of terraced limestone and 200 inhabitants that make a living from farming, fishing and knitwear.
Ruairí de Blacam and his wife, Marie-Thérèse, started the Inis Meáin Knitting Company to earn a livelihood that would allow them to live on the island. The knitting company is by far the island's largest employer, and it makes high quality Irish influenced garments from alpaca, cashmere, Irish linen and merino wool. The first two pieces of theirs that I picked up were hip length linen sweater coats more than a decade ago. I wear them still.
To the best of my knowledge, the only places to buy the knitwear in the United States are Bergdorf Goodman, Saks, or online at Hartford York. But if you're looking for a worthwhile clothing-related adventure, visiting the island gets you to the factory store, which has a range of garments available at what are essentially wholesale prices. I guess their reseller channel doesn't worry about the handful of visitors they get each day during the summer.
The better way to get there is to go to Galway and hop one of the two-a-day ferries out of Ros a'Mhíl (Rossaveel). Or, Aer Arainn Islands operates a commercial air service that takes seven minutes each way. And I'd appreciate it if you'd pick up a couple things for me while you're there.