as....點解??

as 在此處是一個 pronoun, 有 a fact that 的意思, 那個 fact 就是 "the early Irish monks were in fact great seafarers and certainly reached Iceland", 也就是 Norwegian emigrants 所 discovered 的事實. 請看下句:
Indeed, many of the early Irish monks were in fact great seafarers and certainly reached Iceland, a fact that Norwegian emigrants discovered when they first settled there in the eighth century AD.
確然, 很多早期的愛爾蘭僧侶都是偉大的航海家, 亦確實到過冰島. 於公元八世紀移民該地的挪威人正發現了這一點.
as 在上文中並不是 conjunction, 也不是「作為」,「雖然」,「因為」, 或「如此這般」. 用「正如... 一樣」則在意義上是可以的 (正如於公元八世紀移民該地的挪威人所發現的一樣).
舉另一個例子:
He's a foreigner, as is evident from his accent.
He's a foreigner, a fact that is evident from his accent.
他是一個外國人. 這點從他的口音便可得知.

Indeed(實則), many of the early Irish monks were in fact great seafarers and certainly reached Iceland, as (如此這般 JUST) Norwegian emigrants discovered when (當)they first settled there in the eighth century AD.

當挪威人的移民於公元8世紀定居島, 他們發現很多到了島的愛爾蘭僧人實則是大航海家。

類似 because, 用來連接下句作為一個 proof 或者 evidence.

This -as- is a word used to link the 2 parts of the passage.

If used in Chinese a similar word could be 正如

的確, 很多......正如公元八世紀那些挪威移民最初踏足冰島時所發現的一樣

意为"虽然"^_^

＜as ＞在此句的解釋：
作為， 以 Norwegian emigrants ( 移民 ) 的身份

呢個as用作補充many of the early Irish monks were in fact great seafarers and certainly reached Iceland呢句