How to solve this system of trigonometric trancendental equations over the reals?

Sin is defined in the whole complex plane, while ArcSin is not. Solving a system involving ArcSin makes unneccessary problems therefore it is better to use Sin instead ArcSin. Let's take a look at this:

ContourPlot[{x^2 + 2x Sin[y] + 3Cos[y] == 0, x/2 + Sin[y] == Sin[y - Pi/3]}, 
            {x, -25, 25}, {y, -5 Pi, 5 Pi}]

We can see there are infinitely many solutions, however one seems to ask about the only one solution equivalent to that observed with ContourPlot involving ArcSin. Thus we are interested in solutions -4<= x <=4 and -1 <= y - Pi/3 <= 1. We are solving the system without ArcSin (it make the problem easier) however we have to add appropriately modified restriction on variables x and y:

sol = {x, y} /. { ToRules @ Reduce[{x^2 + 2x Sin[y] + 3Cos[y] == 0, 
                                    x/2 + Sin[y] == Sin[y - Pi/3],
                                    -4 <= x <= 4,
                                    -1 <= y - Pi/3 <= 1},
                                   {x, y}]}

{{1/4 (-Sqrt[3] - Sqrt[15]), 2 ArcTan[Sqrt[3/5]]}}

ContourPlot[{x^2 + 2 x Sin[y] + 3 Cos[y] == 0, ArcSin[x/2 + Sin[y]] == y - Pi/3},
            {x, -4, 4}, {y, 0, 2 Pi}, 
    Epilog -> {Red, PointSize[0.025], Point[sol]}, ContourStyle -> Thick,
    PlotPoints -> 25, MaxRecursion -> 4, AspectRatio -> Automatic]

This is an extension of my earlier comment. The solution of the equations, transformed as suggested by b.gatessucks, is

s = Reduce[{x^2 + 2*x*Sin[y] + 3*Cos[y] == 0, 
            x/2 + Sin[y] == Sin[y - Pi/3]}, {x, y}, Reals]
(* (C[1] ∈ Integers && ((x == 1/4 (-Sqrt[3] - Sqrt[15]) && 
   y == 2 ArcTan[Sqrt[3/5]] + 2 π C[1]) || (x == 1/4 (-Sqrt[3] + Sqrt[15]) && 
   y == -2 ArcTan[Sqrt[3/5]] + 2 π C[1]))) || (C[1] ∈ Integers && x == Sqrt[3] && 
   y == π + 2 π C[1]) *)

Now substitute the first of the three solutions into the original equations

FullSimplify[{x^2 + 2*x*Sin[y] + 3*Cos[y] == 0, 
    ArcSin[x/2 + Sin[y]] == y - Pi/3} /. (s[[1, 2, 1]] // ToRules), s[[1, 1]]]
(* {True, C[1] == 0} *)

Therefore, the C[1] == 0 member of the first family of solutions is valid,

({x, y} /. (s[[1, 2, 1]] // ToRules) /. C[1] -> 0)
(* {1/4 (-Sqrt[3] - Sqrt[15]), 2 ArcTan[Sqrt[3/5]]} *)

Doing the same with the second family of solutions, however, yields,

FullSimplify[{x^2 + 2*x*Sin[y] + 3*Cos[y] == 0, 
    ArcSin[x/2 + Sin[y]] == y - Pi/3} /. (s[[1, 2, 2]] // ToRules), s[[1, 1]]]
(* {True, False} *)

and similarly for the third family of solutions.

Note that the one valid solution is equivalent to that from Maple.

FullSimplify[({x, y} /. (s[[1, 2, 1]] // ToRules) /. C[1] -> 0) == 
    {-2 Cos[ArcTan[Sqrt[15]] + Pi/6] - Sqrt[15]/2, ArcTan[Sqrt[15]]}]
(* True *)