Hi, First of all, déanaim comhbhrón leat. (My sympathies) Secondly, as I have said many times here, translating these short phrases from English can be very tricky. To me (and I don't know if this applies where you are) No Fear can mean something like "Not likely!" Does it have this significance where you are and was it a phrase commonly used by your brother? If it was, then there is no way of translating this literally. The closest equivalent would be "Is beag an baol!", which means "It's little the danger!" On reflection, I suppose that is quite similar to No Fear, if you take it to mean No cause for fear.
Otherwise, if we take the phrase to mean just "fearless" or "there is no need to be afraid", then "Gan eagla" would be a reasonable translation.
Hope this helps and again, I am sorry for your loss.