How is a "Y" with an umlaut pronounced?

The diaeresis or trema is the diacritic mark ( ¨ ) used to indicate a phonological diaeresis, or more generally, that a vowel should be pronounced apart from the letter which precedes it. That preceding letter is usually another vowel, but in Spanish it is a consonant. For example, in the spelling coöperate, it reminds the reader that the word has four syllables, not three. In English, the trema is rare, and not mandatory, but other languages like Dutch, Spanish, Brazilian Portuguese and French make regular use of it. By extension, the words trema and diaeresis also designate the same diacritic when used to denote other kinds of sound changes, such as marking the schwa ë in Albanian.

This excerpt focuses on the sounds in relation to the rest of the word.

https://en.wikipedia.org/wiki/%C5%B8

Also from that site:

"Ÿ is sometimes used in transcribed Greek, where it represents the Greek letter υ (upsilon) in the non-diphthong αυ (alpha upsilon) (e.g., in the transcription Artaÿctes of the Persian name Ἀρταΰκτης at the very end of Herodotus). It also occurs in French as a variant of ï, in rare proper nouns (for instance, the name of the Parisian suburb of L'Haÿ-les-Roses).
In Dutch, a handwritten ij can resemble a ÿ (though the latter does not occur in Dutch)."

I might add: It doesn't resemble, it's totally identical.

Being Dutch, I can assure you that there is nothing like the "ij" sound in (standard) English and no way I can describe it without you actually hearing it.

Wiktionary says: "y with a diaeresis, usually used in old texts as a ligature for Dutch or Afrikaans ij: lÿs = lijs, lijst (list)."
http://en.wiktionary.org/wiki/%C3%BF

Very annoying that search engines do not see the difference between y and ÿ.