The number of male calves from the Irish dairy herd has increased following the abolition of EU milk quotas in 2015. This is a new resource for the industry, and they can be reared economically to produce meat of acceptable eating quality with optimal fatty acid (FA) profile. The effects of breed and feeding treatment on the FA profile of the longissimus thoracis (LT) muscles of young dairy bulls (n = 69) of Holstein–Friesian and Jersey × Holstein–Friesian were evaluated. The relationship between FA composition and sensory characteristics was also investigated. Bulls were offered either 1 or 2 kg of concentrates at pasture during the first grazing season. Bulls finished on silage with 5 kg of concentrates or on ad libitum concentrates were slaughtered at 15 months of age, while bulls slaughtered at 19 months were fed either ad libitum pasture or pasture with 5 kg of concentrates during a second grazing season and finished on pasture with 5 kg of concentrates or ad libitum concentrates. Pasture-finished 19-month-old bulls had higher relative proportions of C18:0 (P < 0.001), C20:5n3 (eicosapentaenoic acid; P < 0.001), C18:2n6c (P < 0.05), C20:4n6 (P < 0.01), and total n-3 (P < 0.001) and n-6 polyunsaturated FAs (P < 0.05), but reduced C14:0, C16:0, C14:1, C16:1 and total monounsaturated FA (P < 0.001) proportions than in other production treatments. The silage-based finishing system of 15-month-old bulls increased the relative proportions of C15:0 (P < 0.01), C18:0 (P < 0.001) and total saturated FA (P < 0.01), but decreased the proportions of C20:3n6 (P < 0.05) and total unsaturated FA (P < 0.01) compared with the ad libitum-concentrate finishing system. However, the feeding system of the first or second grazing seasons had limited effects on the FA profile. Jersey × Holstein–Friesian breed type had higher relative proportions of C14:1 and C16:1 (P < 0.05), but lower C15:0 (P < 0.01), C17:0 (P < 0.001) and C17:1 (P < 0.01) proportions than did Holstein–Friesian beef. Slaughter age did not affect the FA profile. Monounsaturated FA, C14:0 and C16:0 proportions were positively (P < 0.05) correlated, but polyunsaturated FA proportion was negatively (P < 0.05) correlated with intramuscular fat content, sensory roast beef aroma, flavour, flavour length, juiciness, initial tenderness and ease of disintegration scores. Finishing system and slaughter weight were well discriminated on the basis of FA relative proportions by canonical discriminant analysis.