In Anglo English, the word ewe (female sheep) is pronounced "you," rather than, say, "e-weh."
Likewise, the surname Ewell, is pronounced "yule," rather than "e-well."
Why is that?
Because of spelling conservatism and sound changes. The word "ewe" is not really pronounced "non phonetically" any more than words like betrayal (which is not "betra-yal") or wither (not "wit-her"). In modern English, "ew"/"eu" simply functions as a digraph that represents the sound /juː/ "yoo."
Digraphs are sequences of two letters that are not pronounced as the sum of their constituent letters. They are used in the spelling systems of a great many languages and arise from various sources. For example, "eu" is also used as a digraph in German (where it represents /ɔʏ/, approximately the vowel in "boy") and French (where it represents /œ/ or /ø/, vowels which have no clear English equivalent but are somewhere in between the vowels in "bet," "bay," "toe," "took," and "fur").
There are many vowel digraphs in English; some of the most common are ay, aw, ow, ew. Many of the digraphs used in English did have more compositional pronunciations in older varieties of the language (for example, claw /klɔː/ comes from Old English clawu /ˈklɑwu/), which turned into the modern pronunciation through processes of regular sound change.
However, there are also many words spelled with digraphs that don't correspond to any historical pronunciation of the word. For example, the digraph ou/ow used to represent /aʊ/ in words like now was taken as a unit from a French digraph ou that represented the sound /u/. The two letters here never represented two distinct sounds in English. The common consonant digraph ch for /tʃ/ also never represented a simple combination of the sounds of the constituent letters "c" and "h."
It turns out that the digraph ew does derive from a representation of Old English diphthongs that simply consisted of an "e"-like sound followed by a "w"-like sound (for example, /ew/ and /eːw). Wikipedia explains that this digraph also came to be used more broadly to represent the sounds that developed from other similar Old English diphthongs such as /æːw/ (as in Old English lǣwede, Modern English lewd) and /iːw/ (as in Old English nīwe, Modern English new). Apparently, all of these sounds developed to /juː/ by Early Modern English through regular sound change.
The word ewe comes from Old English eowu, pronounced something like /ˈeo̯wu/.
I would assume the surname Ewell is related to the placename, which Wikipedia says is "from Old English æwell, which means river source or spring."
Note: although I discuss Old English spelling a bit in this post, be aware that the transcriptions of Old English spelling that I use here are abstractions that don't perfectly represent the forms found in physical Old English texts. In particular, the letter w developed fairly late and in Old English the letter ƿ (called wynn), derived from runic writing, was generally used to represent the sound /w/. It is therefore often transcribed as w in modern transcriptions of Old English. Vowel length was also generally not marked in Old English texts.