Hi there.
You and I are a lot alike! I love kids (have been a nanny for 17 years), but I just don’t know about having my own. However, I’ve been obsessed w/having a plan for what to name one anyway since I was about twelve.
Also, my dh is from a large proud Irish American family, where a ton of great Irish names have been taken by his siblings already. And I also want to use my late dad, [name_m]David[/name_m]’s, name for a mn for a boy. When it comes to such a special mn, I honestly wouldn’t worry too much about fn/mn flow. [name_m]Just[/name_m] pick a fn you love.
Here are my thoughts on your list so far:
Simon [name_m]John[/name_m]-This is adorable. [name_m]Simon[/name_m] is smart, witty, quirky, handsome, classic, simple & not terribly popular. [name_m]Simon[/name_m] [name_m]John[/name_m] sounds very nice.
Colin [name_m]John[/name_m]-it’s a bit rhyme-y, but like I said, not big deal since [name_m]John[/name_m] is so special as a mn. [name_m]Colin[/name_m] is nice. Simple for Americans, but clearly Irish. Sporty, smart & just a good solid name. I kind of prefer [name_m]Cullen[/name_m], since it’s a bit interesting, but also maybe more complicated.
Ewan [name_m]John[/name_m]-[name_m]Ewan[/name_m] is my #1 pick for a boy, & I don’t see him wavering anytime soon. [name_m]Eoin[/name_m] would probably be a headache, I agree. [name_m]Ewan[/name_m] is just beautiful, but in manly way! It’s super uncommon too. I’m not sure how it hasn’t caught on yet, but it’s not even in the top 1000 in the U.S.
Asher [name_m]John[/name_m]-[name_u]Asher[/name_u] is a great name. It sounds outdoorsy & kind of rugged, but soft & gentle at the same time. It’s a little bit more popular, but not super over done I don’t think. [name_u]Asher[/name_u] [name_m]John[/name_m] flows great.
Alasdair [name_m]John[/name_m]-I think that this spelling will be tricky for Americans, but not a deal breaker. A lot of my little Irish named nephews have kind of trickier names, but it’s totally worth it. [name_u]Ali[/name_u] is kind of popular for girls though ([name_u]Allie[/name_u], usually), so I don’t know. I guess it’d be okay for a boy. [name_m]Alasdair[/name_m] [name_m]John[/name_m] sounds perfect, & [name_m]Alasdair[/name_m] is dashing.
Zander [name_m]John[/name_m]-Meh, I don’t love this one. It seems a tad too modern for my tastes. I’d be way more for [name_m]Alexander[/name_m] nn [name_m]Xander[/name_m] personally.
Aran [name_m]John[/name_m]-This will definitely get mispronounced & misspelled, I’d imagine. Although I love the idea of the nn [name_u]Ari[/name_u]. 
Here are a few more ideas that you may like:
Adrian [name_m]John[/name_m]
Atlas [name_m]John[/name_m]
Brennan [name_m]John[/name_m]
Cillian [name_m]John[/name_m]
Ciaran [name_m]John[/name_m]
Casey [name_m]John[/name_m]
Desmond [name_m]John[/name_m]
Eamon [name_m]John[/name_m]
Finnian [name_m]John[/name_m]
Gareth [name_m]John[/name_m]
Koa [name_m]John[/name_m]-a nature name!
Lachlan [name_m]John[/name_m]
Laurence [name_m]John[/name_m]
Malachy [name_m]John[/name_m]
Nolan [name_m]John[/name_m]
Niall [name_m]John[/name_m]
Patrick [name_m]John[/name_m]
Rafferty [name_m]John[/name_m]
River [name_m]John[/name_m]-that works, right?
Silas [name_m]John[/name_m]
Theodore [name_m]John[/name_m]
Tristan [name_m]John[/name_m]
Winston [name_m]John[/name_m]
Wilder [name_m]John[/name_m]
Hope this helps. 