Unless I'm mistaken,when a word ending in a broad s preceeds a word beginning with a slender s they merge and become one slender s eg. "agus Seán" - ag@s'a:n.

So in a phrase like "bhris Sam" (slender s before broad) how would they be pronounced??

Most of the time a broad s doesn't become slender if the following one is slender not the contrary
/ogəs s'æ:n/
/v'r'is' sam/

s > ∫ under the influence of d', t', j, N', L', l', g', k'.

Examples—э∫ d'as is deas, gэ∫ t'ax boxt agus teach bocht, эgi∫ ja:r ∫iэd agus ghearr siad, эgi∫ N'i: mid'i: agus ní muide, ogi∫ L'ag agus leag, э∫ l'at is leat, э∫ g'il'э is gile, э∫ k'uN ós cionn.

When two consonants are identical or become identical in sandhi they coalesce and become a single consonant. The first merges with the second in one articulation. The resultant single consonant has extra length, and for this reason it is written double here and throughout.

Examples—эs m'ihэd dum is mithid damh, э tarэNt ta:rN'i: ag tarrainnt tairngí, э ni:m mir'э a Naomh-Muire, э∫ g'al' l'e∫ is geall leis, fo:∫ ∫e: phós sé, a∫ ∫in' as sin, э tun э huki∫ ∫i: an tan a thiocfas sí, ogi∫ ∫эu i: agus seo í.
...

s replaces ∫ in sandhi in placenames beginning with Inis when the following word begins with a velar, e.g. Inis Gluaire in'эs gluэr'э, Inis Tuirc in'эs tirk', Inis Caorach in'эs ki:rэx.

— Éamonn Mhac an Fhailigh,
"The Irish of Erris, Co. Mayo"

That isn't true for all dialects.

Neither is "Most of the time a broad s doesn't become slender if the following one is slender."