Food coloring is weaker than clothing dye so it would be even less likely to cover up the color you don't want. If you went for black instead of blue you shouldn't have any problems getting the color that you're aiming for.
I'm not sure which stores would have it in Ireland. In the US you can get it at craft stores. Probably some other places too, but I haven't looked for it. Rit is the name of a popular brand. Here's what the box looks like.
