In this paper we examine the economics of the relative return to land use in Ireland when changing from an agricultural system to forestry. To do this we have developed a cost-benefit framework to model the inter-temporal costs and benefits of forestry planting decisions relative to alternative agricultural uses for different environmental and soil conditions. We equate forest productivity to agricultural soil categories and then generate agricultural gross margins for six farm enterprises on six soil types. Using a forest valuation estimator developed for this purpose, the gross margins are included as an opportunity cost in forestry income streams on comparable sites. The results show that forestry does not compete economically with dairy enterprises on any soil type, but that forestry is more profitable than cattle and sheep across all soil types, and particularly on less productive agricultural soils.