Visit to Tashiro Island

I will visit Tashiro island on August 30th with two friends, and I looked up for the ferry timetable.
It looks like there are no trips to go back to Ishinomaki after 15:30, so I was wondering if in August the timetable will change, or if it would be possible to spend the nigh on the island.
Thanks!

Timetable no change on that day. (no operate after 15:30 is right)
This is Ferry official. (Ajishima line)
http://ajishimaline.com/pg117.html
Summer season timetable(夏季ダイヤ 7/18 - 8/16) update yet.

After 15:30 is No way to back Ishinomaki[石巻] port.
So stay "Manga-island[マンガアイランド] (lodges nick)" usual. (need booking)
http://www.city.ishinomaki.lg.jp/cont/10452000b/-kanko/-kankomap/d0040...
No online form but booking[予約] by e-mail possible.
If still wonder use contact[お問い合わせ] form.
https://www.city.ishinomaki.lg.jp/cgi-bin/enquete/enq.cgi
island MAP
http://www.npo-tashirojima.jp/map/index.html

Above booking address provide Ishinomaki city official
but local NPO group more better maybe. (address: bottom of page)
http://www.npo-tashirojima.jp/access/index.html
NPO web mentioned Manga-island with tent(for camper) site info with.
(NPO = Nonprofit Organization)

Or go more far larger Aji-shima[網地島](Aji-island) by last ferry and stay as option.
http://www.i-kanko.com/archives/4834
Click 真崎荘 to 汐亭 for acommodation lower part of hotel/ryokan/minshuku list.

(note)
Tashiro-jima(shima=island) is small no convenience store and restaurant
so bring own foods/drinks before ferry ride if day trip.
Around Ishinomaki ferry port also not convenient (GoogleMAP street-view works)
around Ishinomaki stn or Sendai stn inside (as stop over) better.

Also Ajishima is not so big but about 5 times(x5) conveniet than Tashirojima.
Just one of regualar "human" island.

And around end of Aug already no busy season no worry
but ferry is not big canceled easy
by bad conditions(sea wave/strong wind/heavy fog) be careful.

Well, we are in Japan now, and in 3 days we'll leave for Ishinomaki.
We booked a hotel in Ishinomaki to spend the night and go to Tashiro island first thing the next morning, but since I don't know how the ferry terminal is called I can't find out which train to take to reach it.
We have the Rail pass, so a JR train would be better. Also, the Ferry tickets are to buy on the ferry?
Thanks a lot to anyone who could help!

The ferry terminal is not near the train station. You'll have to take a bus, taxi, or walk it.

Check out this page. The info is in the getting there and around section:

http://www.japan-guide.com/e/e5031.html