slánaitheoir is pronounced \Slaw-Naw-whore\

How does the h after the s affect this pronunciation? One youtube video told me that if a word begins with "Sh" then often times it is silent.

Sh is pronounced h. You can hear shl at http://fuaimeanna.ie/en/Recordings.aspx?Ortho=sh, eg. "mo shliochtach" on that page.

Slánaitheoir is not pronounced /slaw naw .../
Second syllable is not /naw/ but /ni/
Listen: https://www.teanglann.ie/en/fuaim/sl%c3%a1naitheoir

Shl... is pronounced /hl/
The /h/ devoices /l/ so often a voiceless /l/ remains.

It seems that only the Ulster dialect has \ni\ instead of \naw\

Not sure what you mean about "devoices" and so a "voiceless" \l\ remains

There is /ni/ in Connacht https://www.teanglann.ie/en/fuaim/an_Sl%c3%a1naitheoir, too.
Different speakers perhaps.
In Munster, it is in both versions /nə/ (or simplified /nuh/) but never /nɑː/ (/naw/)



Devoicing = unvoicing = "making a sound voiceless".
https://en.wikipedia.org/wiki/Consonant ... _devoicing

There are voiced sounds (e.g. b, g, d, v) and voiceless sounds (p, k, t, f)
Voiced sounds involve the vocal cords, voiceless sounds don’t.

An l sound is usually voiced. You can say a long lllllllllllll without any vowel. The vibration of your vocal cords can be heard.
But you can remove the voice, relax the vocal cords, as well, so llllllllllll is just a whisper or even less. So it is made voiceless.

An h sound (voiceless itself) next to l, n, m, or r (all usually voiced) makes these sounds voiceless – more or less so in many languages but certainly in Irish.*